Reverse Linked List

Reverse a singly linked list.


/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
/*First initialize a pointer 'newHead' pointing to null. In each iteration, we do four steps.
1. New a 'tmp' pointer pointing to 'head.next';
2. Let 'head.next' connect to 'newHead';
3. Let 'newHead' point to 'head';
4. Let 'head' point to 'tmp'.

time: O(n), space:O(1)
*/

public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode newHead = null;
        while (head != null) {
            ListNode tmp = head.next;
            head.next = newHead;
            newHead = head;
            head = tmp;
        }
        return newHead;
    }
}