206. Reverse Linked List

二刷 06/2022

Version #1 Iteration

Time O(N)

Space O(1)

Runtime: 0 ms, faster than 100.00% of Java online submissions for Reverse Linked List.

Memory Usage: 43.5 MB, less than 11.98% of Java online submissions for Reverse Linked List.

class Solution {

    public ListNode reverseList(ListNode head) {

        // 1 -> 2 -> 3

        // <- 1  2 -> 3

        ListNode prev = null;

        ListNode curr = head;

        ListNode next = null;

        while (curr != null) {

            next = curr.next;

            curr.next = prev;

            prev = curr;

            curr = next;

        }

        return prev;

    }

}

一刷
Version #1 Iteration
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode prev = null;
        ListNode curr = head;
        ListNode next = null;
        while (curr != null) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
     
    }
}


Version #2 Recursion
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode newHead = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return newHead;
    }
}