129. Sum Root to Leaf Numbers

class Solution {
    int result = 0;
    public int sumNumbers(TreeNode root) {
        dfs(root, 0);
        return result;
    }
    private void dfs(TreeNode node, int num){
        if (node ==null) return;
        num+=node.val;
        if(node.right==null && node.left ==null){
            result+=num;
        }
        
        dfs(node.left, num*10);
        dfs(node.right, num*10);
    }
}