填充每个节点的下一个右侧节点指针

Populating Next Right Pointers in Each Node

专题: Algorithmic Programming / 算法编程
难度: L3
来源: Citadel

题目详情

问题：填充每个节点的下一个右侧节点指针

考察：树

来源：Citadel

链接：https://www.jointaro.com/interviews/questions/populating-next-right-pointers-in-each-node/

You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition:

struct Node {
  int val;
  Node *left;
  Node *right;
  Node *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.

Example 1:

Input: root = [1,2,3,4,5,6,7]
Output: [1,#,2,3,#,4,5,6,7,#]
Explanation: Given the above perfect binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B. The serialized output is in level order as connected by the next pointers, with '#' signifying the end of each level.

Example 2:

Input: root = []
Output: []

Constraints:

The number of nodes in the tree is in the range [0, 2¹² - 1].
-1000 <= Node.val <= 1000

Follow-up:

You may only use constant extra space.
The recursive approach is fine. You may assume implicit stack space does not count as extra space for this problem.

解析

思路：完美二叉树可利用已建立的 next 指针逐层连接：node.left.next = node.right，node.right.next = node.next.left。逐层从最左节点向右遍历。

复杂度：时间 O(n)，额外空间 O(1)。