【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)