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 = Noneclass 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)