leetcode中等143重排链表

Posted 2022-03-29 qq_40707462

给定一个单链表 L 的头节点 head ，单链表 L 表示为：

L0 → L1 → … → Ln - 1 → Ln
请将其重新排列后变为：

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

/**
 * Definition for singly-linked list.
 * public class ListNode 
 *     int val;
 *     ListNode next;
 *     ListNode() 
 *     ListNode(int val)  this.val = val; 
 *     ListNode(int val, ListNode next)  this.val = val; this.next = next; 
 * 
 */
class Solution 
    public void reorderList(ListNode head) 
        //1、返回中点（偏前）
        ListNode mid=split(head);
        ListNode l1=head;
        ListNode l2=mid.next;
        mid.next=null;
        //2、反转l2
        l2=reverse(l2);
        //3、合并
        merge(l1,l2);
    
    public ListNode split(ListNode head)
        ListNode fast=head,slow=head;
        while(fast.next!=null&&fast.next.next!=null)
            slow=slow.next;
            fast=fast.next.next;
        
        return slow;
    
    public ListNode reverse(ListNode head)
        ListNode pre=null,cur=head;
        while(cur!=null)
            ListNode next=cur.next;
            cur.next=pre;
            pre=cur;
            cur=next;
        
        return pre;
    
    public void merge(ListNode l1,ListNode l2)
        ListNode t1,t2;
        while(l1!=null&&l2!=null)
            t1=l1.next;
            t2=l2.next;

            l1.next=l2;
            l1=t1;
            l2.next=l1;
            l2=t2;