Leetcode: Sum Root to Leaf Numbers

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   /   2   3
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

 1 /**
 2  * Definition for binary tree
 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 int sumNumbers(TreeNode root) {
12         if (root == null) return 0;
13         ArrayList<Integer> set = new ArrayList<Integer>();
14         helper(set, 0, root);
15         int sum = 0;
16         for (int elem : set) {
17             sum = sum + elem;
18         }
19         return sum;
20     }
21     
22     public void helper(ArrayList<Integer> set, int num, TreeNode n) {
23         if (n.left == null && n.right == null) {
24             num = num*10 + n.val;
25             set.add(num);
26             return;
27         }
28         else {
29             num = num*10 + n.val;
30             if (n.left != null) helper(set, num, n.left);
31             if (n.right != null) helper(set, num, n.right);
32         }
33     }
34 }

 1 public int sumNumbers(TreeNode root) {
 2     return helper(root,0);
 3 }
 4 private int helper(TreeNode root, int sum)
 5 {
 6     if(root == null)
 7         return 0;
 8     if(root.left==null && root.right==null)
 9         return sum*10+root.val;
10     return helper(root.left,sum*10+root.val)+helper(root.right,sum*10+root.val);
11 }