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.
1 用的递归的方法：root为空时，返回False；没有左右子树，且root.val==sum时，返回True；其余情况分左右子树来讨论，当然sum要随之变成sum-root.val，当走到叶子节点的时候，没有左右子树了，如果这时叶子节点的值等于此时target的值，则返回True，否则返回False
