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)