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.

For example,

  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.

Java

public class Solution {
    public int sumNumbers(TreeNode root) {
        return sumNumbers(root, 0);
    }

    private int sumNumbers(TreeNode root, int sum) {
        if(root == null) {
            return 0;
        }

        sum = sum * 10 + root.val;

        return (root.left == null && root.right == null) ? sum : sumNumbers(root.left, sum) + sumNumbers(root.right, sum);
    }
}