515. Find Largest Value in Tree Row - Explanation

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def largestValues(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []

        res = []
        q = deque([root])
        while q:
            row_max = q[0].val
            for _ in range(len(q)):
                node = q.popleft()
                row_max = max(row_max, node.val)
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)
            res.append(row_max)

        return res

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def largestValues(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []

        res = []
        def dfs(node, level):
            if not node:
                return
            if level == len(res):
                res.append(node.val)
            else:
                res[level] = max(res[level], node.val)

            dfs(node.left, level + 1)
            dfs(node.right, level + 1)

        dfs(root, 0)
        return res

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def largestValues(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []

        res = []
        stack = [(root, 0)]
        while stack:
            node, level = stack.pop()
            if level == len(res):
                res.append(node.val)
            else:
                res[level] = max(res[level], node.val)

            if node.right:
                stack.append((node.right, level + 1))
            if node.left:
                stack.append((node.left, level + 1))

        return res