LeetCode 206. Reverse Linked List

reverse-linked-list.java

/**
 * https://leetcode.com/problems/reverse-linked-list/
 *
 * O(n) runtime, O(1) space
 *
 * this question can be solved by using an iteration method or a recursion method
 */

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

// iteration
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode cur = head;
        head = null;
        while (cur != null) {
            ListNode next = cur.next;
            cur.next = head;
            head = cur;
            cur = next;
        }
        return head;
    }
}

// recursion
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode next = head.next;
        ListNode newHead = reverseList(next); // recursively reverse the sub list with nodes after the current node and return ites head
        next.next = head; // next must be the last node in the reversed sub list
                          // add the current node after the reversed sub list
        head.next = null; // cut off the link between the current node and nodes after it
        return newHead; // return the new head
    }
}

great

nice !