Given a singly linked list L: L 0→L 1→…→L n-1→L n, reorder it to: L 0→L n →L 1→L n-1→L 2→L n-2→…-蒲公英云

Given a singly linked list L: L 0→L 1→…→L n-1→L n, reorder it to: L 0→L n →L 1→L n-1→L 2→L n-2→…

Bertha 。 2022-05-26 04:25 302阅读 0赞

题目描述

Given a singly linked list L: L 0→L 1→…→L n-1→L n,
reorder it to: L 0→L n →L 1→L n-1→L 2→L n-2→…
You must do this in-place without altering the nodes’ values.(在不改变结点值的情况下)
For example,
Given{1,2,3,4}, reorder it to{1,4,2,3}.

思路：
快慢指针找到中间结点，然后根据中间结点，将链表断开，翻转后面部分链表，然后将两个链表合并

/** * Definition for singly-linked list. * class ListNode { * int val; * ListNode next; * ListNode(int x) { * val = x; * next = null; * } * } */
public class Solution { 
    public void reorderList(ListNode head) {
        if(head == null || head.next == null || head.next.next == null)
            return;
        ListNode slow = head;
        ListNode fast = head;
        //快慢指针找到中间结点
        while(fast != null && fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        //断开链表，fast指向后半部分链表，head仍然是头结点，之前前半部分链表
        fast = slow;
        slow = slow.next;
        fast.next = null;
        //翻转链表
        ListNode lastHead = ReverseNode(slow);
        ListNode newHead = head;
        head = head.next;
        //合并链表
        while(head != null && lastHead != null){
            newHead.next = lastHead;
            lastHead = lastHead.next;
            newHead = newHead.next;
            newHead.next = head;
            head = head.next;
            newHead = newHead.next;
        }
        if(head != null)
            newHead.next = head;
        if(lastHead != null)
            newHead.next = lastHead;
    }
    private ListNode ReverseNode(ListNode head){
        ListNode prev = null;
        ListNode curr = head;
        while(curr != null && curr.next != null){
            ListNode next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        curr.next = prev;
        return curr;
    }
}