LeetCode Roots [129] to the leaf node sum of the numbers

Title:
Given a binary tree, each node that are stored in a digital 0-9, from the root to each leaf node represents a number of paths.

For example, the path from the root to the leaf node 1-> 2-> 3 represents the number 123.

Calculated from the root to the leaf nodes generated by the sum of all the numbers.

Description: leaf node is a node has no child nodes.

Example 1:

Input: [1,2,3]
1
/
2 3
Output: 25
Explanation:
from the root to a leaf node of the path 1-> 2 represents a number 12
from the root to a leaf node of the path 1-> 3 represents the number 13.
Thus, the digital sum = 12 + 13 = 25.

class Solution {

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