Path Sum

Problem

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,
return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

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Idea

dfs遍历每条路径. 注意: 节点可以是负数。 只有到了叶子的时候才判断长度是否为sum

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Solution

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