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.

/**
 * 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root == null){
            return 0;
        }
        ArrayList<String> result = new ArrayList<String>();
        StringBuilder sb = new StringBuilder();
        int sum = 0;
        dfs(root, sb, result);
        for(int i = 0; i < result.size(); i++){
            sum += Integer.parseInt(result.get(i));
        }
        return sum;
    }
    
    public void dfs(TreeNode root, StringBuilder sb, ArrayList<String> result){
        if(root.left == null && root.right == null){
            sb.append(root.val);
            result.add(sb.toString());
            sb.deleteCharAt(sb.length() - 1);
            return;
        }
        sb.append(root.val);
        if(root.left != null){
            //sb.append(root.left.val);
            dfs(root.left, sb, result);
            //sb.deleteCharAt(sb.length() - 1);
        }
        if(root.right != null){
            //sb.append(root.right.val);
            dfs(root.right, sb, result);
            //sb.deleteCharAt(sb.length() - 1);
        }
        sb.deleteCharAt(sb.length() - 1);
    }
}