Leetcode #129 Sum Root to Leaf Numbers

[yokostan]

Genius Recursive:

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

    private int helper(TreeNode root, int sum) {
        if (root == null) return 0;
        sum = sum*10 + root.val;
        if (root.left == null && root.right == null) return sum;
        return helper(root.left, sum) + helper(root.right, sum);
    }
}