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
  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.

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumNumbers(TreeNode root) {
        return sum(root, 0);
    }
    
    private int sum(TreeNode root, int total) {
        if (root == null) return 0;
        int value = total * 10 + root.val;
        if (root.left != null || root.right != null) {
            return sum(root.left, value) + sum(root.right, value);
        } else {
            return value;
        }
    }
}