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"]

 解法: 继续用深度搜索的方法。注意这里的递归过程，各自数据结构之间的转换。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> res = new ArrayList<String>();
        if(root == null){
            return res;
        }
        String cur = new String();
        cur += Integer.toString(root.val);
        helper(root, res, cur);
        return res;
    }
    public void helper(TreeNode root, List<String> res, String cur){
        if(root == null) return;
        if(root.left == null && root.right == null){
            res.add(new String(cur));
            return;
        }
        if(root.left != null){
            helper(root.left, res, cur + "->" + Integer.toString(root.left.val));
        }
        if(root.right != null){
            helper(root.right, res, cur + "->" + Integer.toString(root.right.val));
        }
    }
}