sum-of-left-leaves_404.py

# 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.


# Example 1:


# Input: root = [3,9,20,null,null,15,7]
# Output: 24
# Explanation: There are two left leaves in the binary tree, with values 9 and 15 respectively.
# Example 2:

# Input: root = [1]
# Output: 0

# Constraints:

# The number of nodes in the tree is in the range [1, 1000].
# -1000 <= Node.val <= 1000

# ---------------------------------------Runtime 37 ms Beats 51.72% Memory 16.70 MB Beats 57.71%---------------------------------------

from typing import Optional


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:
        self.res = 0

        def dfs(node: Optional[TreeNode], flag: bool = False) -> None:
            """
            left - True
            right - False
            """
            if node:
                self.res += (
                    flag and not node.left and not node.right and node.val
                )
                dfs(node.left, True)
                dfs(node.right, False)

        dfs(root)
        return self.res