1463. Cherry Pickup II - Explanation

Prerequisites

Before attempting this problem, you should be comfortable with:

Dynamic Programming - Using memoization or tabulation to solve optimization problems with overlapping subproblems
2D/3D Arrays - Working with multi-dimensional data structures for state representation
Recursion with Multiple Variables - Tracking multiple entities (two robots) simultaneously through recursive state exploration

1. Recursion

Intuition

Two robots start at the top corners of the grid and move down simultaneously, one row at a time. At each row, each robot can move diagonally left, straight down, or diagonally right. We track both robot positions and explore all 9 combinations of moves (3 choices for robot 1 times 3 choices for robot 2). If both robots land on the same cell, we only count the cherries once. We enforce c1 <= c2 to avoid duplicate states since the robots are interchangeable.

Algorithm

Define a recursive function dfs(r, c1, c2) where r is the current row, c1 is robot 1's column, and c2 is robot 2's column.
Base case: If c1 or c2 is out of bounds, or c1 > c2, return 0.
Base case: If r equals ROWS - 1, return the sum of cherries at both positions (counting once if they overlap).
For each of the 9 direction combinations (c1_d, c2_d in [-1, 0, 1]):
- Recursively call dfs(r + 1, c1 + c1_d, c2 + c2_d).
- Track the maximum result.
Add cherries from current positions to the result and return.

class Solution:
    def cherryPickup(self, grid: List[List[int]]) -> int:
        ROWS, COLS = len(grid), len(grid[0])

        def dfs(r, c1, c2):
            if c1 < 0 or c2 < 0 or c1 >= COLS or c2 >= COLS or c1 > c2:
                return 0
            if r == ROWS - 1:
                return grid[r][c1] + (grid[r][c2] if c1 != c2 else 0)

            res = 0
            for c1_d in [-1, 0, 1]:
                for c2_d in [-1, 0, 1]:
                    res = max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d))

            return res + grid[r][c1] + (grid[r][c2] if c1 != c2 else 0)

        return dfs(0, 0, COLS - 1)

public class Solution {
    public int cherryPickup(int[][] grid) {
        int ROWS = grid.length, COLS = grid[0].length;

        return dfs(0, 0, COLS - 1, grid, ROWS, COLS);
    }

    private int dfs(int r, int c1, int c2, int[][] grid, int ROWS, int COLS) {
        if (c1 < 0 || c2 < 0 || c1 >= COLS || c2 >= COLS || c1 > c2) {
            return 0;
        }
        if (r == ROWS - 1) {
            return grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
        }

        int res = 0;
        for (int c1_d = -1; c1_d <= 1; c1_d++) {
            for (int c2_d = -1; c2_d <= 1; c2_d++) {
                res = Math.max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d, grid, ROWS, COLS));
            }
        }
        return res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
    }
}

class Solution {
public:
    int cherryPickup(vector<vector<int>>& grid) {
        int ROWS = grid.size(), COLS = grid[0].size();
        return dfs(0, 0, COLS - 1, grid, ROWS, COLS);
    }

private:
    int dfs(int r, int c1, int c2, vector<vector<int>>& grid, int ROWS, int COLS) {
        if (c1 < 0 || c2 < 0 || c1 >= COLS || c2 >= COLS || c1 > c2) {
            return 0;
        }
        if (r == ROWS - 1) {
            return grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
        }

        int res = 0;
        for (int c1_d = -1; c1_d <= 1; c1_d++) {
            for (int c2_d = -1; c2_d <= 1; c2_d++) {
                res = max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d, grid, ROWS, COLS));
            }
        }
        return res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
    }
};

class Solution {
    /**
     * @param {number[][]} grid
     * @return {number}
     */
    cherryPickup(grid) {
        const ROWS = grid.length,
            COLS = grid[0].length;

        const dfs = (r, c1, c2) => {
            if (c1 < 0 || c2 < 0 || c1 >= COLS || c2 >= COLS || c1 > c2)
                return 0;
            if (r === ROWS - 1) {
                return grid[r][c1] + (c1 === c2 ? 0 : grid[r][c2]);
            }

            let res = 0;
            for (let c1_d = -1; c1_d <= 1; c1_d++) {
                for (let c2_d = -1; c2_d <= 1; c2_d++) {
                    res = Math.max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d));
                }
            }
            return res + grid[r][c1] + (c1 === c2 ? 0 : grid[r][c2]);
        };

        return dfs(0, 0, COLS - 1);
    }
}

public class Solution {
    private int ROWS, COLS;
    private int[][] grid;

    public int CherryPickup(int[][] grid) {
        this.grid = grid;
        ROWS = grid.Length;
        COLS = grid[0].Length;
        return Dfs(0, 0, COLS - 1);
    }

    private int Dfs(int r, int c1, int c2) {
        if (c1 < 0 || c2 < 0 || c1 >= COLS || c2 >= COLS || c1 > c2) {
            return 0;
        }
        if (r == ROWS - 1) {
            return grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
        }

        int res = 0;
        for (int c1_d = -1; c1_d <= 1; c1_d++) {
            for (int c2_d = -1; c2_d <= 1; c2_d++) {
                res = Math.Max(res, Dfs(r + 1, c1 + c1_d, c2 + c2_d));
            }
        }
        return res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
    }
}

class Solution {
    fun cherryPickup(grid: Array<IntArray>): Int {
        val ROWS = grid.size
        val COLS = grid[0].size

        fun dfs(r: Int, c1: Int, c2: Int): Int {
            if (c1 < 0 || c2 < 0 || c1 >= COLS || c2 >= COLS || c1 > c2) {
                return 0
            }
            if (r == ROWS - 1) {
                return grid[r][c1] + if (c1 == c2) 0 else grid[r][c2]
            }

            var res = 0
            for (c1_d in -1..1) {
                for (c2_d in -1..1) {
                    res = maxOf(res, dfs(r + 1, c1 + c1_d, c2 + c2_d))
                }
            }
            return res + grid[r][c1] + if (c1 == c2) 0 else grid[r][c2]
        }

        return dfs(0, 0, COLS - 1)
    }
}

class Solution {
    func cherryPickup(_ grid: [[Int]]) -> Int {
        let ROWS = grid.count
        let COLS = grid[0].count

        func dfs(_ r: Int, _ c1: Int, _ c2: Int) -> Int {
            if c1 < 0 || c2 < 0 || c1 >= COLS || c2 >= COLS || c1 > c2 {
                return 0
            }
            if r == ROWS - 1 {
                return grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2])
            }

            var res = 0
            for c1_d in -1...1 {
                for c2_d in -1...1 {
                    res = max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d))
                }
            }
            return res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2])
        }

        return dfs(0, 0, COLS - 1)
    }
}

Time & Space Complexity

Time complexity: $O(m * 9 ^ n)$
Space complexity: $O(n)$ for recursion stack.

Where $n$ is the number of rows and $m$ is the number of columns in the grid.

2. Dynamic Programming (Top-Down)

Intuition

The recursive solution recomputes the same states multiple times. Since the state is defined by (r, c1, c2), we can use a 3D cache to store results for previously visited states. When we encounter a state we have already solved, we return the cached value instead of recomputing it.

Algorithm

Create a 3D cache array of size [ROWS][COLS][COLS] initialized to -1.
In the recursive function, first check if the state is already in the cache. If so, return the cached value.
Perform the same logic as the recursive solution: handle base cases, try all 9 move combinations, and compute the maximum.
Before returning, store the computed result in the cache.
Return the cached result.

class Solution:
    def cherryPickup(self, grid: List[List[int]]) -> int:
        ROWS, COLS = len(grid), len(grid[0])
        cache = {}

        def dfs(r, c1, c2):
            if (r, c1, c2) in cache:
                return cache[(r, c1, c2)]
            if c1 == c2 or min(c1, c2) < 0 or max(c1, c2) >= COLS:
                return 0
            if r == ROWS - 1:
                return grid[r][c1] + grid[r][c2]

            res = 0
            for c1_d in [-1, 0, 1]:
                for c2_d in [-1, 0, 1]:
                    res = max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d))

            cache[(r, c1, c2)] = res + grid[r][c1] + (grid[r][c2] if c1 != c2 else 0)
            return cache[(r, c1, c2)]

        return dfs(0, 0, COLS - 1)

public class Solution {
    private int[][][] cache;
    public int cherryPickup(int[][] grid) {
        int ROWS = grid.length, COLS = grid[0].length;
        cache = new int[ROWS][COLS][COLS];
        for (int[][] i : cache) {
            for (int[] j : i) {
                Arrays.fill(j, -1);
            }
        }

        return dfs(0, 0, COLS - 1, grid);
    }

    private int dfs(int r, int c1, int c2, int[][] grid) {
        if (Math.min(c1, c2) < 0 || Math.max(c1, c2) >= grid[0].length) {
            return 0;
        }
        if (cache[r][c1][c2] != -1) {
            return cache[r][c1][c2];
        }
        if (r == grid.length - 1) {
            return cache[r][c1][c2] = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
        }

        int res = 0;
        for (int c1_d = -1; c1_d <= 1; c1_d++) {
            for (int c2_d = -1; c2_d <= 1; c2_d++) {
                res = Math.max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d, grid));
            }
        }
        return cache[r][c1][c2] = res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
    }
}

class Solution {
    vector<vector<vector<int>>> cache;

public:
    int cherryPickup(vector<vector<int>>& grid) {
        int ROWS = grid.size(), COLS = grid[0].size();
        cache.assign(ROWS, vector<vector<int>>(COLS, vector<int>(COLS, -1)));
        return dfs(0, 0, COLS - 1, grid);
    }

private:
    int dfs(int r, int c1, int c2, vector<vector<int>>& grid) {
        if (min(c1, c2) < 0 || max(c1, c2) >= grid[0].size()) {
            return 0;
        }
        if (cache[r][c1][c2] != -1) {
            return cache[r][c1][c2];
        }
        if (r == grid.size() - 1) {
            return cache[r][c1][c2] = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
        }

        int res = 0;
        for (int c1_d = -1; c1_d <= 1; c1_d++) {
            for (int c2_d = -1; c2_d <= 1; c2_d++) {
                res = max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d, grid));
            }
        }
        return cache[r][c1][c2] = res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
    }
};

class Solution {
    /**
     * @param {number[][]} grid
     * @return {number}
     */
    cherryPickup(grid) {
        const ROWS = grid.length,
            COLS = grid[0].length;
        const cache = Array.from({ length: ROWS }, () =>
            Array.from({ length: COLS }, () => Array(COLS).fill(-1)),
        );

        const dfs = (r, c1, c2) => {
            if (Math.min(c1, c2) < 0 || Math.max(c1, c2) >= COLS) {
                return 0;
            }
            if (cache[r][c1][c2] !== -1) return cache[r][c1][c2];
            if (r === ROWS - 1) {
                return (cache[r][c1][c2] =
                    grid[r][c1] + (c1 === c2 ? 0 : grid[r][c2]));
            }

            let res = 0;
            for (let c1_d = -1; c1_d <= 1; c1_d++) {
                for (let c2_d = -1; c2_d <= 1; c2_d++) {
                    res = Math.max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d));
                }
            }
            return (cache[r][c1][c2] =
                res + grid[r][c1] + (c1 === c2 ? 0 : grid[r][c2]));
        };

        return dfs(0, 0, COLS - 1);
    }
}

public class Solution {
    private int[,,] cache;
    private int[][] grid;

    public int CherryPickup(int[][] grid) {
        this.grid = grid;
        int ROWS = grid.Length, COLS = grid[0].Length;
        cache = new int[ROWS, COLS, COLS];
        for (int i = 0; i < ROWS; i++) {
            for (int j = 0; j < COLS; j++) {
                for (int k = 0; k < COLS; k++) {
                    cache[i, j, k] = -1;
                }
            }
        }
        return Dfs(0, 0, COLS - 1, ROWS, COLS);
    }

    private int Dfs(int r, int c1, int c2, int ROWS, int COLS) {
        if (Math.Min(c1, c2) < 0 || Math.Max(c1, c2) >= COLS) {
            return 0;
        }
        if (cache[r, c1, c2] != -1) {
            return cache[r, c1, c2];
        }
        if (r == ROWS - 1) {
            return cache[r, c1, c2] = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
        }

        int res = 0;
        for (int c1_d = -1; c1_d <= 1; c1_d++) {
            for (int c2_d = -1; c2_d <= 1; c2_d++) {
                res = Math.Max(res, Dfs(r + 1, c1 + c1_d, c2 + c2_d, ROWS, COLS));
            }
        }
        return cache[r, c1, c2] = res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
    }
}

func cherryPickup(grid [][]int) int {
    ROWS, COLS := len(grid), len(grid[0])
    cache := make([][][]int, ROWS)
    for i := range cache {
        cache[i] = make([][]int, COLS)
        for j := range cache[i] {
            cache[i][j] = make([]int, COLS)
            for k := range cache[i][j] {
                cache[i][j][k] = -1
            }
        }
    }

    var dfs func(r, c1, c2 int) int
    dfs = func(r, c1, c2 int) int {
        if min(c1, c2) < 0 || max(c1, c2) >= COLS {
            return 0
        }
        if cache[r][c1][c2] != -1 {
            return cache[r][c1][c2]
        }
        if r == ROWS-1 {
            if c1 == c2 {
                cache[r][c1][c2] = grid[r][c1]
            } else {
                cache[r][c1][c2] = grid[r][c1] + grid[r][c2]
            }
            return cache[r][c1][c2]
        }

        res := 0
        for c1_d := -1; c1_d <= 1; c1_d++ {
            for c2_d := -1; c2_d <= 1; c2_d++ {
                res = max(res, dfs(r+1, c1+c1_d, c2+c2_d))
            }
        }
        if c1 == c2 {
            cache[r][c1][c2] = res + grid[r][c1]
        } else {
            cache[r][c1][c2] = res + grid[r][c1] + grid[r][c2]
        }
        return cache[r][c1][c2]
    }

    return dfs(0, 0, COLS-1)
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}

class Solution {
    fun cherryPickup(grid: Array<IntArray>): Int {
        val ROWS = grid.size
        val COLS = grid[0].size
        val cache = Array(ROWS) { Array(COLS) { IntArray(COLS) { -1 } } }

        fun dfs(r: Int, c1: Int, c2: Int): Int {
            if (minOf(c1, c2) < 0 || maxOf(c1, c2) >= COLS) {
                return 0
            }
            if (cache[r][c1][c2] != -1) {
                return cache[r][c1][c2]
            }
            if (r == ROWS - 1) {
                cache[r][c1][c2] = grid[r][c1] + if (c1 == c2) 0 else grid[r][c2]
                return cache[r][c1][c2]
            }

            var res = 0
            for (c1_d in -1..1) {
                for (c2_d in -1..1) {
                    res = maxOf(res, dfs(r + 1, c1 + c1_d, c2 + c2_d))
                }
            }
            cache[r][c1][c2] = res + grid[r][c1] + if (c1 == c2) 0 else grid[r][c2]
            return cache[r][c1][c2]
        }

        return dfs(0, 0, COLS - 1)
    }
}

class Solution {
    func cherryPickup(_ grid: [[Int]]) -> Int {
        let ROWS = grid.count
        let COLS = grid[0].count
        var cache = Array(repeating: Array(repeating: Array(repeating: -1, count: COLS), count: COLS), count: ROWS)

        func dfs(_ r: Int, _ c1: Int, _ c2: Int) -> Int {
            if min(c1, c2) < 0 || max(c1, c2) >= COLS {
                return 0
            }
            if cache[r][c1][c2] != -1 {
                return cache[r][c1][c2]
            }
            if r == ROWS - 1 {
                cache[r][c1][c2] = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2])
                return cache[r][c1][c2]
            }

            var res = 0
            for c1_d in -1...1 {
                for c2_d in -1...1 {
                    res = max(res, dfs(r + 1, c1 + c1_d, c2 + c2_d))
                }
            }
            cache[r][c1][c2] = res + grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2])
            return cache[r][c1][c2]
        }

        return dfs(0, 0, COLS - 1)
    }
}

Time & Space Complexity

Time complexity: $O(n * m ^ 2)$
Space complexity: $O(n * m ^ 2)$

Where $n$ is the number of rows and $m$ is the number of columns in the grid.

3. Dynamic Programming (Bottom-Up)

Intuition

Instead of using recursion with memoization, we can build the solution iteratively from the bottom row up. For each cell combination at row r, we look at all possible transitions from row r+1 and take the maximum. This eliminates recursion overhead and makes the computation order explicit.

Algorithm

Create a 3D DP array of size [ROWS][COLS][COLS].
Process rows from bottom to top (r = ROWS-1 to 0).
For each (c1, c2) pair at row r:
- Calculate cherries at current positions (handling overlap).
- If at the last row, the value is just the current cherries.
- Otherwise, look at all 9 possible (nc1, nc2) positions in row r+1.
- Take the maximum from valid next positions and add current cherries.
Store the result in dp[r][c1][c2].
Return dp[0][0][COLS-1].

class Solution:
    def cherryPickup(self, grid: List[List[int]]) -> int:
        ROWS, COLS = len(grid), len(grid[0])
        dp = [[[0] * COLS for _ in range(COLS)] for _ in range(ROWS)]

        for r in range(ROWS - 1, -1, -1):
            for c1 in range(COLS):
                for c2 in range(COLS):
                    res = grid[r][c1]
                    if c1 != c2:
                        res += grid[r][c2]

                    if r != ROWS - 1:
                        max_cherries = 0
                        for d1 in [-1, 0, 1]:
                            for d2 in [-1, 0, 1]:
                                nc1, nc2 = c1 + d1, c2 + d2
                                if 0 <= nc1 < COLS and 0 <= nc2 < COLS:
                                    max_cherries = max(max_cherries, dp[r + 1][nc1][nc2])
                        res += max_cherries

                    dp[r][c1][c2] = res

        return dp[0][0][COLS - 1]

public class Solution {
    public int cherryPickup(int[][] grid) {
        int ROWS = grid.length, COLS = grid[0].length;
        int[][][] dp = new int[ROWS][COLS][COLS];

        for (int r = ROWS - 1; r >= 0; r--) {
            for (int c1 = 0; c1 < COLS; c1++) {
                for (int c2 = 0; c2 < COLS; c2++) {
                    int res = grid[r][c1];
                    if (c1 != c2) {
                        res += grid[r][c2];
                    }

                    if (r != ROWS - 1) {
                        int maxCherries = 0;
                        for (int d1 = -1; d1 <= 1; d1++) {
                            for (int d2 = -1; d2 <= 1; d2++) {
                                int nc1 = c1 + d1, nc2 = c2 + d2;
                                if (nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS) {
                                    maxCherries = Math.max(maxCherries, dp[r + 1][nc1][nc2]);
                                }
                            }
                        }
                        res += maxCherries;
                    }

                    dp[r][c1][c2] = res;
                }
            }
        }

        return dp[0][0][COLS - 1];
    }
}

class Solution {
public:
    int cherryPickup(vector<vector<int>>& grid) {
        int ROWS = grid.size(), COLS = grid[0].size();
        int dp[ROWS][COLS][COLS];

        for (int r = ROWS - 1; r >= 0; r--) {
            for (int c1 = 0; c1 < COLS; c1++) {
                for (int c2 = 0; c2 < COLS; c2++) {
                    int res = grid[r][c1];
                    if (c1 != c2) {
                        res += grid[r][c2];
                    }

                    if (r != ROWS - 1) {
                        int maxCherries = 0;
                        for (int d1 = -1; d1 <= 1; d1++) {
                            for (int d2 = -1; d2 <= 1; d2++) {
                                int nc1 = c1 + d1, nc2 = c2 + d2;
                                if (nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS) {
                                    maxCherries = max(maxCherries, dp[r + 1][nc1][nc2]);
                                }
                            }
                        }
                        res += maxCherries;
                    }

                    dp[r][c1][c2] = res;
                }
            }
        }

        return dp[0][0][COLS - 1];
    }
};

class Solution {
    /**
     * @param {number[][]} grid
     * @return {number}
     */
    cherryPickup(grid) {
        const ROWS = grid.length,
            COLS = grid[0].length;
        const dp = Array.from({ length: ROWS }, () =>
            Array.from({ length: COLS }, () => Array(COLS).fill(0)),
        );

        for (let r = ROWS - 1; r >= 0; r--) {
            for (let c1 = 0; c1 < COLS; c1++) {
                for (let c2 = 0; c2 < COLS; c2++) {
                    let res = grid[r][c1];
                    if (c1 !== c2) {
                        res += grid[r][c2];
                    }

                    if (r !== ROWS - 1) {
                        let maxCherries = 0;
                        for (let d1 = -1; d1 <= 1; d1++) {
                            for (let d2 = -1; d2 <= 1; d2++) {
                                const nc1 = c1 + d1,
                                    nc2 = c2 + d2;
                                if (
                                    nc1 >= 0 &&
                                    nc1 < COLS &&
                                    nc2 >= 0 &&
                                    nc2 < COLS
                                ) {
                                    maxCherries = Math.max(
                                        maxCherries,
                                        dp[r + 1][nc1][nc2],
                                    );
                                }
                            }
                        }
                        res += maxCherries;
                    }

                    dp[r][c1][c2] = res;
                }
            }
        }

        return dp[0][0][COLS - 1];
    }
}

public class Solution {
    public int CherryPickup(int[][] grid) {
        int ROWS = grid.Length, COLS = grid[0].Length;
        int[,,] dp = new int[ROWS, COLS, COLS];

        for (int r = ROWS - 1; r >= 0; r--) {
            for (int c1 = 0; c1 < COLS; c1++) {
                for (int c2 = 0; c2 < COLS; c2++) {
                    int res = grid[r][c1];
                    if (c1 != c2) {
                        res += grid[r][c2];
                    }

                    if (r != ROWS - 1) {
                        int maxCherries = 0;
                        for (int d1 = -1; d1 <= 1; d1++) {
                            for (int d2 = -1; d2 <= 1; d2++) {
                                int nc1 = c1 + d1, nc2 = c2 + d2;
                                if (nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS) {
                                    maxCherries = Math.Max(maxCherries, dp[r + 1, nc1, nc2]);
                                }
                            }
                        }
                        res += maxCherries;
                    }

                    dp[r, c1, c2] = res;
                }
            }
        }

        return dp[0, 0, COLS - 1];
    }
}

class Solution {
    fun cherryPickup(grid: Array<IntArray>): Int {
        val ROWS = grid.size
        val COLS = grid[0].size
        val dp = Array(ROWS) { Array(COLS) { IntArray(COLS) } }

        for (r in ROWS - 1 downTo 0) {
            for (c1 in 0 until COLS) {
                for (c2 in 0 until COLS) {
                    var res = grid[r][c1]
                    if (c1 != c2) {
                        res += grid[r][c2]
                    }

                    if (r != ROWS - 1) {
                        var maxCherries = 0
                        for (d1 in -1..1) {
                            for (d2 in -1..1) {
                                val nc1 = c1 + d1
                                val nc2 = c2 + d2
                                if (nc1 in 0 until COLS && nc2 in 0 until COLS) {
                                    maxCherries = maxOf(maxCherries, dp[r + 1][nc1][nc2])
                                }
                            }
                        }
                        res += maxCherries
                    }

                    dp[r][c1][c2] = res
                }
            }
        }

        return dp[0][0][COLS - 1]
    }
}

class Solution {
    func cherryPickup(_ grid: [[Int]]) -> Int {
        let ROWS = grid.count
        let COLS = grid[0].count
        var dp = Array(repeating: Array(repeating: Array(repeating: 0, count: COLS), count: COLS), count: ROWS)

        for r in stride(from: ROWS - 1, through: 0, by: -1) {
            for c1 in 0..<COLS {
                for c2 in 0..<COLS {
                    var res = grid[r][c1]
                    if c1 != c2 {
                        res += grid[r][c2]
                    }

                    if r != ROWS - 1 {
                        var maxCherries = 0
                        for d1 in -1...1 {
                            for d2 in -1...1 {
                                let nc1 = c1 + d1
                                let nc2 = c2 + d2
                                if nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS {
                                    maxCherries = max(maxCherries, dp[r + 1][nc1][nc2])
                                }
                            }
                        }
                        res += maxCherries
                    }

                    dp[r][c1][c2] = res
                }
            }
        }

        return dp[0][0][COLS - 1]
    }
}

Time & Space Complexity

Time complexity: $O(n * m ^ 2)$
Space complexity: $O(n * m ^ 2)$

Where $n$ is the number of rows and $m$ is the number of columns in the grid.

4. Dynamic Programming (Space Optimized)

Intuition

Since each row's computation only depends on the row below it, we do not need to store the entire 3D array. We can use two 2D arrays: one for the current row and one for the previous row (the row below). After processing each row, we swap them. This reduces space complexity from O(n * m^2) to O(m^2).

Algorithm

Create a 2D dp array of size [COLS][COLS] for the previous row's results.
Process rows from bottom to top.
For each row, create a new 2D cur_dp array for the current row.
For each (c1, c2) pair where c1 <= c2:
- Calculate cherries at current positions.
- Look at all 9 next positions in the dp array (previous row).
- Take the maximum and add current cherries.
After processing the row, set dp = cur_dp.
Return dp[0][COLS-1].

class Solution:
    def cherryPickup(self, grid: List[List[int]]) -> int:
        ROWS, COLS = len(grid), len(grid[0])
        dp = [[0] * COLS for _ in range(COLS)]

        for r in reversed(range(ROWS)):
            cur_dp = [[0] * COLS for _ in range(COLS)]
            for c1 in range(COLS):
                for c2 in range(c1, COLS):
                    max_cherries = 0
                    cherries = grid[r][c1] + (grid[r][c2] if c1 != c2 else 0)
                    for d1 in [-1, 0, 1]:
                        for d2 in [-1, 0, 1]:
                            nc1, nc2 = c1 + d1, c2 + d2
                            if 0 <= nc1 < COLS and 0 <= nc2 < COLS:
                                max_cherries = max(max_cherries, cherries + dp[nc1][nc2])
                    cur_dp[c1][c2] = max_cherries
            dp = cur_dp

        return dp[0][COLS - 1]

public class Solution {
    public int cherryPickup(int[][] grid) {
        int ROWS = grid.length, COLS = grid[0].length;
        int[][] dp = new int[COLS][COLS];

        for (int r = ROWS - 1; r >= 0; r--) {
            int[][] cur_dp = new int[COLS][COLS];
            for (int c1 = 0; c1 < COLS; c1++) {
                for (int c2 = c1; c2 < COLS; c2++) {
                    int maxCherries = 0;
                    int cherries = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
                    for (int d1 = -1; d1 <= 1; d1++) {
                        for (int d2 = -1; d2 <= 1; d2++) {
                            int nc1 = c1 + d1, nc2 = c2 + d2;
                            if (nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS) {
                                maxCherries = Math.max(maxCherries, cherries + dp[nc1][nc2]);
                            }
                        }
                    }
                    cur_dp[c1][c2] = maxCherries;
                }
            }
            dp = cur_dp;
        }
        return dp[0][COLS - 1];
    }
}

class Solution {
public:
    int cherryPickup(vector<vector<int>>& grid) {
        int ROWS = grid.size(), COLS = grid[0].size();
        vector<vector<int>> dp(COLS, vector<int>(COLS, 0));

        for (int r = ROWS - 1; r >= 0; r--) {
            vector<vector<int>> cur_dp(COLS, vector<int>(COLS, 0));
            for (int c1 = 0; c1 < COLS; c1++) {
                for (int c2 = c1; c2 < COLS; c2++) {
                    int maxCherries = 0;
                    int cherries = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
                    for (int d1 = -1; d1 <= 1; d1++) {
                        for (int d2 = -1; d2 <= 1; d2++) {
                            int nc1 = c1 + d1, nc2 = c2 + d2;
                            if (nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS) {
                                maxCherries = max(maxCherries, cherries + dp[nc1][nc2]);
                            }
                        }
                    }
                    cur_dp[c1][c2] = maxCherries;
                }
            }
            dp = cur_dp;
        }
        return dp[0][COLS - 1];
    }
};

class Solution {
    /**
     * @param {number[][]} grid
     * @return {number}
     */
    cherryPickup(grid) {
        const ROWS = grid.length,
            COLS = grid[0].length;
        let dp = Array.from({ length: COLS }, () => Array(COLS).fill(0));

        for (let r = ROWS - 1; r >= 0; r--) {
            const cur_dp = Array.from({ length: COLS }, () =>
                Array(COLS).fill(0),
            );
            for (let c1 = 0; c1 < COLS; c1++) {
                for (let c2 = c1; c2 < COLS; c2++) {
                    let maxCherries = 0;
                    const cherries =
                        grid[r][c1] + (c1 === c2 ? 0 : grid[r][c2]);
                    for (let d1 = -1; d1 <= 1; d1++) {
                        for (let d2 = -1; d2 <= 1; d2++) {
                            const nc1 = c1 + d1,
                                nc2 = c2 + d2;
                            if (
                                nc1 >= 0 &&
                                nc1 < COLS &&
                                nc2 >= 0 &&
                                nc2 < COLS
                            ) {
                                maxCherries = Math.max(
                                    maxCherries,
                                    cherries + dp[nc1][nc2],
                                );
                            }
                        }
                    }
                    cur_dp[c1][c2] = maxCherries;
                }
            }
            dp = cur_dp;
        }

        return dp[0][COLS - 1];
    }
}

public class Solution {
    public int CherryPickup(int[][] grid) {
        int ROWS = grid.Length, COLS = grid[0].Length;
        int[,] dp = new int[COLS, COLS];

        for (int r = ROWS - 1; r >= 0; r--) {
            int[,] cur_dp = new int[COLS, COLS];
            for (int c1 = 0; c1 < COLS; c1++) {
                for (int c2 = c1; c2 < COLS; c2++) {
                    int maxCherries = 0;
                    int cherries = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2]);
                    for (int d1 = -1; d1 <= 1; d1++) {
                        for (int d2 = -1; d2 <= 1; d2++) {
                            int nc1 = c1 + d1, nc2 = c2 + d2;
                            if (nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS) {
                                maxCherries = Math.Max(maxCherries, cherries + dp[nc1, nc2]);
                            }
                        }
                    }
                    cur_dp[c1, c2] = maxCherries;
                }
            }
            dp = cur_dp;
        }
        return dp[0, COLS - 1];
    }
}

class Solution {
    fun cherryPickup(grid: Array<IntArray>): Int {
        val ROWS = grid.size
        val COLS = grid[0].size
        var dp = Array(COLS) { IntArray(COLS) }

        for (r in ROWS - 1 downTo 0) {
            val cur_dp = Array(COLS) { IntArray(COLS) }
            for (c1 in 0 until COLS) {
                for (c2 in c1 until COLS) {
                    var maxCherries = 0
                    val cherries = grid[r][c1] + if (c1 == c2) 0 else grid[r][c2]
                    for (d1 in -1..1) {
                        for (d2 in -1..1) {
                            val nc1 = c1 + d1
                            val nc2 = c2 + d2
                            if (nc1 in 0 until COLS && nc2 in 0 until COLS) {
                                maxCherries = maxOf(maxCherries, cherries + dp[nc1][nc2])
                            }
                        }
                    }
                    cur_dp[c1][c2] = maxCherries
                }
            }
            dp = cur_dp
        }
        return dp[0][COLS - 1]
    }
}

class Solution {
    func cherryPickup(_ grid: [[Int]]) -> Int {
        let ROWS = grid.count
        let COLS = grid[0].count
        var dp = Array(repeating: Array(repeating: 0, count: COLS), count: COLS)

        for r in stride(from: ROWS - 1, through: 0, by: -1) {
            var cur_dp = Array(repeating: Array(repeating: 0, count: COLS), count: COLS)
            for c1 in 0..<COLS {
                for c2 in c1..<COLS {
                    var maxCherries = 0
                    let cherries = grid[r][c1] + (c1 == c2 ? 0 : grid[r][c2])
                    for d1 in -1...1 {
                        for d2 in -1...1 {
                            let nc1 = c1 + d1
                            let nc2 = c2 + d2
                            if nc1 >= 0 && nc1 < COLS && nc2 >= 0 && nc2 < COLS {
                                maxCherries = max(maxCherries, cherries + dp[nc1][nc2])
                            }
                        }
                    }
                    cur_dp[c1][c2] = maxCherries
                }
            }
            dp = cur_dp
        }

        return dp[0][COLS - 1]
    }
}

Time & Space Complexity

Time complexity: $O(n * m ^ 2)$
Space complexity: $O(m ^ 2)$

Where $n$ is the number of rows and $m$ is the number of columns in the grid.

Common Pitfalls

Double Counting Cherries When Robots Overlap

Forgetting to check if both robots land on the same cell and counting cherries twice. When c1 == c2, you must only add grid[r][c1] once, not grid[r][c1] + grid[r][c2].

# Wrong: always adds both
res = grid[r][c1] + grid[r][c2]

# Correct: check for overlap
res = grid[r][c1] + (grid[r][c2] if c1 != c2 else 0)

Using 4 Directions Instead of 3

Treating this like a general grid traversal where robots can move in any direction. Each robot can only move down-left, down, or down-right (3 choices), not up or stay in place. This gives 9 total combinations per step, not 16.

Incorrect State Space for Memoization

Using only (r, c1) or (r, c2) for caching, forgetting that the state depends on both robot positions simultaneously. The correct state is (r, c1, c2) since both robots move together row by row.

Off-by-One Error on Base Case

Checking r == ROWS instead of r == ROWS - 1 for the base case. Since indexing is 0-based, the last row is at index ROWS - 1, and that is where you should collect final cherries and stop recursion.