[LC] 129. 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.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
    1
   / \
  2   3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int res = 0;
    public int sumNumbers(TreeNode root) {
        helper(root, 0);
        return res;
    }
    
    private void helper(TreeNode root, int sum) {
        if (root == null) {
            return;
        }
        int curSum = 10 * sum + root.val;
        if (root.left == null && root.right == null) {
            res += curSum;
        }
        helper(root.left, curSum);
        helper(root.right, curSum);
    }
}