Sum Root to Leaf Numbers

终于完成了LeetCode 上AC率在30%以上的题目，这道题是最后一道，纪念一下。

原题是这个样子的：

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.

解题思路：递归

代码如下：

public class SumRoottoLeafNumbers {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, prev * 10 + root.val)+ sum(root.right, prev * 10 + root.val);} else if (root.left != null) {return sum(root.left, prev * 10 + root.val);} else {return sum(root.right, prev * 10 + root.val);}}}