LeetCode Populating Next Right Pointers in Each Node

原题链接在这里：https://leetcode.com/problems/populating-next-right-pointers-in-each-node/

题目：

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

题解：

递归调用，如果root.next不为null, root.right.next 等于 root.next.left, 然后root.left.next指向root.right.

Time Complexity: O(n), 每个点访问不超过两遍. Space: O(logn).

AC Java:

/**
* 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;
}
if(root.right != null && root.next != null){
root.right.next = root.next.left;
}
if(root.left!=null){
root.left.next = root.right;
}
connect(root.right);
connect(root.left);
}
}

Iteration版本

Time Complexity: O(n). Space: O(1).

/**
* 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) {
TreeLinkNode head = root;
while(head != null){
TreeLinkNode nextDummy = new TreeLinkNode(0);
TreeLinkNode cur = nextDummy;
while(head != null){
if(head.left != null){
cur.next = head.left;
cur = cur.next;
}
if(head.right != null){
cur.next = head.right;
cur = cur.next;
}
head = head.next;
}
head = nextDummy.next;
}
}
}

跟上Populating Next Right Pointers in Each Node II