404. Sum of Left Leaves.py

# Find the sum of all left leaves in a given binary tree.

# Example:

#     3
#    / \
#   9  20
#     /  \
#    15   7

# There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

# DFS

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.res = 0
        self.helper(root, False)
        return self.res
    def helper(self, root, isLeft):
        if not root:
            return 
        if not root.left and not root.right and isLeft:
            self.res += root.val
        self.helper(root.left, True)
        self.helper(root.right, False)