Leetcode 112 Path Sum

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.

Tree经典题,使得加起来和等于sum的条件就是左／右子树存在和值加上root的值等于sum。终止条件为root为null -> false 或者root不存在左右子树且达到sum -> true。

class Solution:
def hasPathSum(self, root, sum):
if not root:
return False
if not root.left and not root.right and root.val == sum:
return True
return self.hasPathSum(root.left, sum-root.val) or self.hasPathSum(root.right, sum-root.val)