LeetCode 129. Sum Root to Leaf Numbers（节点求和）

原题网址：https://leetcode.com/problems/sum-root-to-leaf-numbers/

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.

方法：递归。

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