129. Sum Root to Leaf Numbers

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumNumbers(TreeNode root) {
        return sumNumbersHelp(root, 0);
    }
    public int sumNumbersHelp(TreeNode root, int currentSum){
        if(root == null){
            return 0;
        }
        int newSum = addNumberToSum(currentSum, root.val);
        
        if(isLeaf(root)){
            return newSum;
        }
        else{//Not a leaf
            return sumNumbersHelp(root.left, newSum) + sumNumbersHelp(root.right, newSum);
        }
        
    }
    
    public int addNumberToSum(int sum, int number){
        return (sum * 10) + number;
    }
    
    public boolean isLeaf(TreeNode root){
        return root.left == null && root.right == null;
    }
}