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.
Note: A leaf is a node with no children.
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.
减去当前节点值的只，然后递归调用，到叶子节点和目标值相等即可。
注意把代码写简化点！
这道题和 129. Sum Root to Leaf Numbers 类似。
link:{sourcedir}/_0112_PathSum.java[role=include]