【leetcode】Path Sum

题目简述：

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:
Given the below binary tree and sum = 22,
5
/
4 8
/ /
11 13 4
/
7 2 1
return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

解题思路：

注意这里的路径必须是从根到叶子（必须到叶子！）。这里有个神奇的现象sum -= root.val这句不谢的话，下面改成if sum == root.val and root.left == None and root.right == None:就过不了

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # @param root, a tree node
    # @param sum, an integer
    # @return a boolean
    def hasPathSum(self, root, sum):
        ret = False
        if root == None:
            return ret
        sum -= root.val
        if sum == 0 and root.left == None and root.right == None:
            ret = True
        return ret or self.hasPathSum(root.left,sum) or self.hasPathSum(root.right,sum)