257. Binary Tree Paths

 1     public List<String> binaryTreePaths(TreeNode root) {
 2         List<String> res = new ArrayList<String>();
 3         if(root == null) {
 4             return res;
 5         }
 6         traversal(res, "", root);
 7         return res;
 8     }
 9     
10     private void traversal(List<String> res, String cur, TreeNode root) {
11         if(root == null) {
12             return;
13         }
14         cur += "->";
15         cur += String.valueOf(root.val);
16         if(root.left == null && root.right == null) {
17             cur = cur.substring(2);
18             res.add(cur);
19             return;
20         }
21         traversal(res, cur, root.left);
22         traversal(res, cur, root.right);
23     }