leetcode 257[easy]-----Binary Tree Paths

难度：easy

Given a binary tree, return all root-to-leaf paths.

For example, given the following binary tree:

   1 /   \2     3 \  5

All root-to-leaf paths are:

["1->2->5", "1->3"]

思路：开始学习两叉树，仍然是以练带学，边练边学的方式。用递归的方法完成。