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);
		}
	}
}

Sum Root to Leaf Numbers