116. Populating Next Right Pointers in Each Node

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

思路：
定义了三个指针，first、pre、current。
first 指向每一行的起始位置
pre 指向父节点
current 指向当前节点

根据current和pre的关系，可以做出如下判断：
1. current == pre.left 则直接让current.next = pre.right
2. current == pre.right 可以分为两种情况：
    (1) pre.next == NULL
      例如：当pre指向1并且current指向3时，1的next为NULL，此时fisrt、pre均下移到2、3那一行。
    (2) pre.next != NULL
      例如：当pre指向2并且current指向5时，此时5的next应指向6

根据上述的思路，程序如下：

/** * Definition for binary tree with next pointer. * public class TreeLinkNode { *     int val; *     TreeLinkNode left, right, next; *     TreeLinkNode(int x) { val = x; } * } */public class Solution {    public void connect(TreeLinkNode root) {        if(root == null) return ;        TreeLinkNode pre = root;        TreeLinkNode current = pre.left;        TreeLinkNode first = root;        while(current != null) {            if(current == pre.left) {                current.next = pre.right;            }else if(current == pre.right) {                if(pre.next != null) {                    current.next = pre.next.left;                       pre = pre.next;                }else {                    first = first.left;                    pre = first;                    current = pre.left;                     continue;                }            }            current = current.next;        }    }}