题目

• 原题连接：143. 重排链表

1- 思路

• 模式识别：重排链表 ——> 逆向 ——> ① 找到中间节点 ——> ②逆置 mid.next 链表——> ③遍历

2- 实现

⭐143. 重排链表——题解思路

class Solution {
    public void reorderList(ListNode head) {

        ListNode mid = findMiddle(head);

        ListNode leftHead = head;
        ListNode rightHead = mid.next;
        mid.next = null;

        rightHead = reverList(rightHead);
        
        while(leftHead!=null && rightHead!=null){

            // 先记录两链表的下一个结点
            ListNode leftNext = leftHead.next;
            ListNode rightNext = rightHead.next;

            leftHead.next = rightHead;
            leftHead = leftNext;

            rightHead.next = leftHead;
            rightHead = rightNext;
        }


    }

    public ListNode findMiddle(ListNode head){
        ListNode fast = head;
        ListNode slow = head;

        while(fast!=null && fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    public ListNode reverList(ListNode head){
        if(head==null || head.next==null) return head;

        ListNode cur = reverList(head.next);

        head.next.next = head;
        head.next = null;
        return cur;
    }
}

3- ACM 实现

public class linkResort {


    static class ListNode{
        int val;
        ListNode next;
        ListNode(){}
        ListNode(int x){
            val = x;
        }
    }

    public static ListNode linkSort(ListNode head){
        ListNode mid  = findMid(head);

        ListNode leftHead = head;
        ListNode rightHead = mid.next;
        mid.next = null;
        rightHead = reverse(rightHead);

        while (leftHead!=null && rightHead!=null) {
            ListNode leftNext = leftHead.next;
            ListNode rightNext = rightHead.next;

            leftHead.next = rightHead;
            leftHead = leftNext;

            rightHead.next = leftHead;
            rightHead = rightNext;
        }

        return head;
    }


    public static ListNode findMid(ListNode head){
        ListNode slow = head;
        ListNode fast = head;
        while(fast!=null && fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    public static ListNode reverse(ListNode head){
        if(head== null || head.next==null) {
            return head;
        }
        ListNode cur = reverse(head.next);

        head.next.next = head;
        head.next = null;
        return cur;
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("输入链表长度n");
        int n = sc.nextInt();
        ListNode head=null,tail=null;
        System.out.println("输入链表元素");
        for(int i = 0 ; i < n; i++){
            ListNode newNode = new ListNode(sc.nextInt());
            if(head==null){
                head = newNode;
                tail = newNode;
            }else{
                tail.next = newNode;
                tail = newNode;
            }
        }

        ListNode forRes = linkSort(head);
        while(forRes!=null){
            System.out.print(forRes.val+" ");
            forRes = forRes.next;
        }
    }
}