Leetcode-116. Populating Next Right Pointers in Each Node

前言：为了后续的实习面试，开始疯狂刷题，非常欢迎志同道合的朋友一起交流。因为时间比较紧张，目前的规划是先过一遍，写出能想到的最优算法，第二遍再考虑最优或者较优的方法。如有错误欢迎指正。博主首发CSDN，mcf171专栏。

博客链接：mcf171的博客

——————————————————————————————

Given a binary tree

    struct TreeLinkNode {      TreeLinkNode *left;      TreeLinkNode *right;      TreeLinkNode *next;    }

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Note:

• You may only use constant extra space.
• You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

For example,
Given the following perfect binary tree,

         1       /  \      2    3     / \  / \    4  5  6  7

After calling your function, the tree should look like:

         1 -> NULL       /  \      2 -> 3 -> NULL     / \  / \    4->5->6->7 -> NULL

这个题目让我深切意识到了自己对于边界条件思考的不周密，以及逻辑上考虑的欠妥。错了3、4次才对。Your runtime beats 6.86% of java submissions.

public class Solution {    public void connect(TreeLinkNode root) {        Queue<TreeLinkNode> queue = new LinkedList<TreeLinkNode>();        queue.offer(root);        connect(queue);    }    public void connect(Queue<TreeLinkNode> queue){        int level = 0;        while(queue.peek() !=null){            TreeLinkNode leftNode = queue.poll();            queue.offer(leftNode.left);            queue.offer(leftNode.right);            int number = (int)Math.pow(2,level);            for(int i = 0 ; i < number; i ++){                if( i == number - 1)leftNode.next = null;                else{                    TreeLinkNode rightNode = queue.poll();                    leftNode.next = rightNode;                    leftNode = rightNode;                    queue.offer(leftNode.left);                    queue.offer(leftNode.right);                }            }            level ++;        }    }}

感觉效率有点低，看了下论坛，发现备受打击，感觉算法真有点检验智商，其次是自己考虑周全吗，虽然自己总结是广度优先是用队列，原因是跨子树无法相互访问，但是这个题目是很容易跨子树相互访问的。。Your runtime beats 76.82% of java submissions.

public class Solution {    public void connect(TreeLinkNode root) {TreeLinkNode level_start = root;while(level_start != null && level_start.left != null){TreeLinkNode node = level_start;while(node.next != null){node.left.next = node.right;node.right.next = node.next.left;node = node.next;}node.left.next = node.right;level_start = level_start.left;}    }}