[LeetCode]Sum of Left Leaves

题目描述：

LeetCode 404. Sum of Left Leaves

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.

题目大意：

给定二叉树，计算其所有左叶子的和。

解题思路：

遍历二叉树

Python代码：

# 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
        """
        ans = 0
        if root: 
            l, r = root.left, root.right
            if l and (l.left or l.right) is None:
                ans += l.val
            ans += self.sumOfLeftLeaves(l) + self.sumOfLeftLeaves(r)
        return ans