404. Sum of Left Leaves

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

# 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:
        res = 0
        def preorder(root):
            nonlocal res
            if root is None:
                return
            if root.left and (not root.left.left and not root.left.right):
                res += root.left.val
            preorder(root.left)
            preorder(root.right)
        preorder(root)
        return res