Reverse Linked List

Reverse a singly linked list.

Example:

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

Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

(Iterative)

• Time complexity : O(n). Assume that n is the list's length, the time complexity is O(n)

• Space complexity : O(1).

/**
 * 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){
            return head;
        }
        
        ListNode pre = null;
        ListNode next = null;
        ListNode cur = head;
        
        while(cur != null){
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        
        return pre;
    }
}

(Recursive)

• Time complexity : O(n). Assume that is the list's length, the time complexity is O(n).

• Space complexity : O(n). The extra space comes from implicit stack space due to recursion. The recursion could go up to n levels deep.

public ListNode reverseList(ListNode head) {
  if(head == null || head.next == null) return head;
        ListNode p = reverseList(head.next);
        head.next.next= head;
        head.next = null;
        return p;
  }