Sum Root to Leaf Numbers.java

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.

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