[LeetCode] 404. Sum of Left Leaves

 Problem : https://leetcode.com/problems/sum-of-left-leaves/

Use post-order traversal to accumulate left leave values.

# 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:
        
        self.result = 0
        
        def postorder(node):
            if not node.left and not node.right:
                # find a leaf
                return True
            
            if node.left and postorder(node.left):
                # find a left leaf
                self.result += node.left.val
            
            if node.right:
                postorder(node.right)
            
            return False
        
        postorder(root)
        return self.result