Binary Tree Paths leetcode

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"]

public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> res = new ArrayList<String>();
        if (root == null) {
            return res;
        }
        String str = root.val +"";
        helper(res, root, str);
        return res;
    }
    public void helper(List<String> res, TreeNode root, String str) {
        if (root.left == null && root.right == null) {
            res.add(str);
            return;
        }
        if (root.left != null) {
            helper(res, root.left, str + "->" + root.left.val);
        }
        if (root.right != null) {
            helper(res, root.right, str +"->" + root.right.val);
        }
    }
}