Sum Root to Leaf Numbers

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    List<String> num_list = new ArrayList<String>();
    
    public void DFS(TreeNode root,String num) {
        if(root!=null) {
            if(root.left==null&&root.right==null) num_list.add(num+root.val);
            if(root.left!=null) DFS(root.left,num+root.val);
            if(root.right!=null) DFS(root.right,num+root.val);
        }
    }
    
    public int sumNumbers(TreeNode root) {
        String num = "";
        DFS(root,num);
        int sum = 0;
        for(String s:num_list) {
            sum = sum+Integer.valueOf(s);
        }
        return sum;
    }
}