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.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(root == null) return 0;
        dfs(result, 0, root);
        int sum = 0;
        for(int i : result){
            sum += i;
        }
        
        return sum;
    }
    
    private void dfs(ArrayList<Integer> result, int currentNumber, TreeNode root){
        if(root.left == null && root.right == null){
            result.add(10*currentNumber+root.val);
        }
        
        currentNumber = currentNumber*10+root.val;
        
        if(root.left != null){
            dfs(result, currentNumber, root.left);
        }
        if(root.right != null){
            dfs(result, currentNumber, root.right);
        }
　　     // number是值传递，所以不用-1
    }
}