[The sword refers to the offer brushing the question] AcWing 35. Reverse linked list (simple, double pointer, recursion)

Question type: double pointer, recursion

1. It can be written with double pointers, which is easier to think and simpler
2. It can also be solved in a recursive way, pay attention to the way of thinking of recursive programming

topic

Define a function that takes as input the head node of a linked list, reverses the linked list and outputs the head node of the reversed linked list.

Thinking questions:

请同时实现迭代版本和递归版本。

Sample

Input: 1->2->3->4->5->NULL

Output: 5->4->3->2->1->NULL

java code

recursive solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    public ListNode reverseList(ListNode head) {
    
    
        if (head == null || head.next == null) return head;

        ListNode tail = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return tail;
    }
}

iterative solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    
    
    public ListNode reverseList(ListNode head) {
    
    
        if (head == null || head.next == null) return head;

        ListNode pre = head, q = head.next;
        while (q != null) {
    
    
            ListNode t = q.next;
            q.next = pre;
            pre = q;
            q = t;
        }

        head.next = null;
        return pre;
    }
}