2965. Find Missing and Repeated Values - Explanation

class Solution:
    def findMissingAndRepeatedValues(self, grid: List[List[int]]) -> List[int]:
        n = len(grid)
        double = missing = 0

        for num in range(1, n * n + 1):
            cnt = 0
            for i in range(n):
                for j in range(n):
                    if num == grid[i][j]:
                        cnt += 1

            if cnt == 2:
                double = num
            elif cnt == 0:
                missing = num

        return [double, missing]

class Solution:
    def findMissingAndRepeatedValues(self, grid: List[List[int]]) -> List[int]:
        N = len(grid)
        count = defaultdict(int)

        for i in range(N):
            for j in range(N):
                count[grid[i][j]] += 1

        double = missing = 0

        for num in range(1, N * N + 1):
            if count[num] == 0:
                missing = num
            if count[num] == 2:
                double = num

        return [double, missing]

class Solution:
    def findMissingAndRepeatedValues(self, grid: List[List[int]]) -> List[int]:
        N = len(grid)
        seen = set()
        double = missing = 0

        for i in range(N):
            for j in range(N):
                if grid[i][j] in seen:
                    double = grid[i][j]
                seen.add(grid[i][j])

        for num in range(1, N * N + 1):
            if num not in seen:
                missing = num
                break

        return [double, missing]

class Solution:
    def findMissingAndRepeatedValues(self, grid: List[List[int]]) -> List[int]:
        N = len(grid)
        gridSum = 0
        gridSqSum = 0

        for i in range(N):
            for j in range(N):
                gridSum += grid[i][j]
                gridSqSum += grid[i][j] * grid[i][j]

        totSum = (N * N * (N * N + 1)) // 2
        diff = gridSum - totSum  # a - b

        totSqSum = (N * N * (N * N + 1) * (2 * N * N + 1)) // 6
        sqDiff = gridSqSum - totSqSum  # (a^2) - (b^2)

        sum = sqDiff // diff  # a + b

        a = (sum + diff) // 2
        b = sum - a
        return [a, b]