Given a binary tree, return all root-to-leaf paths.For example, given the following binary tree:   1 /   \2     3 \  5All root-to-leaf paths are:["1->2->5", "1->3"]

 

1 /** 2  * Definition for a binary tree node. 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public List
     
       binaryTreePaths(TreeNode root) {12         List
      
        res = new ArrayList
       
        ();13         if (root == null) return res;14         helper(root, res, "");15         return res;16     }17     18     public void helper(TreeNode root, List
        
          res, String item) {19         if (root.left == null && root.right == null) {20             if (item.length() == 0) {21                 res.add("" + root.val);22             }23             else {24                 res.add(item + "->" + root.val);25             }26             return;27         }28         if (root.left != null) 29             helper(root.left, res, item.length()==0? ""+root.val : item+"->"+root.val);30         if (root.right != null)31             helper(root.right, res, item.length()==0? ""+root.val : item+"->"+root.val);32         33     }34 }