404. Sum of Left Leaves

# one code: 36ms, 95%; 39ms, 77%; 36ms, 95% 
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

# going through the tree: [3] --> [9], 1 (<- 9) --> [20], 0 --> [15] ,1 (<-- 15) --> [7],0  (<-- 0)--> None, 1 (<-- 0) --> None 
class Solution(object):
    def helper (self, root, flag):
        if root == None:
            return 0
        if root.left == None and root.right == None and flag == 1:
            return root.val
        result = self.helper (root.left, 1)
        result += self.helper (root.right, 0)
        return result
    
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        # how to determine it is left leaf? --> root.left == None and root.right == None and flag == 1--> this is left leaf,             #return value
        # self.helper (root.left, 1) , self.helper (root.right, 0)
        if root == None:
            return 0
        result = self.helper (root.left, 1)
        result += self.helper (root.right, 0)
        return result