[LeetCode] Path Sum 求二叉树中满足要求的路径

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.

题目意思是说给定一个二叉树后判断是否存在从根到叶的一条路径，让该路径上的接点之和为sum，最开始想，把所有的路径保存下来，这样肯定是不对的，后来慢慢想到了递归，可以这么想，给定sum，如果root->val != sum， 则继续判断root->left->val，如此类推，这个地方注意，题目要求是“从根到叶子的路径”这个地方错了好几次，最后代码如下

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