Leetcode 404. Sum of Left Leaves

Problem

Given the root of a binary tree, return the sum of all left leaves.
A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

Algorithm

Recursion. Use a flag mark it when goes to left sub-tree. When it comes from a left sub-tree and get to the leaf state, we sum the value.

Code

# 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 sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        def dfs(root, s):
            if not root.left and not root.right:
                if s == 1:
                    return root.val
                else:
                    return 0
            ans = 0
            if root.left:
                ans += dfs(root.left, 1)
            if root.right:
                ans += dfs(root.right, 0)
            return ans
        
        return dfs(root, 0)