LeetCode 206 Reverse Linked List

Posted 2020-08-20

Reverse a singly linked list. 

Solution 1 (recursion)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
 

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null|| head.next ==null)
        {
            return head;
        }
        ListNode second = head.next;
        //set first‘s next to be null
        head.next = null;
 
        ListNode rest = ReverseList(second);
        second.next = head;
 
    return rest;
    }
}

Solution 2 Iteration

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null|| head.next ==null)
        {
            return head;
        }
        ListNode prev = null;
        while(head!=null)
        {
            ListNode temp = head.next;
            head.next = prev;
            prev= head;
            head = temp;
        }
        return prev;
    }
}