Leetcode: Binary Tree Paths

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"]

/**
 * 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;
        helper(root, res, "");
        return res;
    }
    
    public void helper(TreeNode root, List<String> res, String item) {
        if (root.left == null && root.right == null) {
            if (item.length() == 0) {
                res.add("" + root.val);
            }
            else {
                res.add(item + "->" + root.val);
            }
            return;
        }
        if (root.left != null) 
            helper(root.left, res, item.length()==0? ""+root.val : item+"->"+root.val);
        if (root.right != null)
            helper(root.right, res, item.length()==0? ""+root.val : item+"->"+root.val);
        
    }
}