[LeetCode] 129. Sum Root to Leaf Numbers Java

题目：

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.

题意及分析：给出一棵二叉树，每个节点的值均为0-9，根节点到叶子节点上的值组成一个数字，遍历树，求根节点到所有叶子结点的数字之和。使用先序遍历，用一个变量count储存到每个非叶子结点得到的值，对于每个叶子结点将父节点的值*10+本节点的值就得到根节点到该叶子结点的值，加到和sum里面即可。

代码：

/**
 * Definition for a binary tree node.
 * 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;
        int[] nums = new int[1];     //保存sum的值,
        build(nums,root,0);
        return nums[0];
    }

    public void build(int[] sum,TreeNode node,int temp){
        int count = temp;
        if(node.left==null&&node.right==null){      //叶子节点计算值
            sum[0] += count*10+node.val;
            return;
        }
        count = count*10+node.val;        //记录每个非叶子节点的值
        if(node.left!=null){
            build(sum,node.left,count);
        }
        if(node.right!=null){
            build(sum,node.right,count);
        }
    }
}