404. 左叶子之和

题目链接：https://leetcode-cn.com/problems/sum-of-left-leaves/
难度：简单

描述：
给定二叉树的根节点 root ，返回所有左叶子之和。

题解

# 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, is_left_node=False):
            ret = 0
            if root is None:
                return ret
            if is_left_node and root.left is None and root.right is None:
                ret += root.val
            if root.left:
                ret += dfs(root.left, True)
            if root.right:
                ret += dfs(root.right, False)
            return ret
        if root is None:
            return 0
        return dfs(root)