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.
题目只要求返回true或者false，因此不需要记录路径。
由于只需要求出一个结果，因此，当左、右任意一棵子树求到了满意结果，都可以及时return。
由于题目没有说节点的数据一定是正整数，必须要走到叶子节点才能判断，因此中途没法剪枝，只能进行朴素深搜。
{% codesnippet "./code/path-sum."+book.suffix, language=book.suffix %}{% endcodesnippet %}