[linked list] reverse linked list

✅ Problem

https://leetcode.com/problems/reverse-linked-list/description/

 

✅ Approach & Solution

방식1) iterative

• 현재 노드의 next가 이전 노드여야 한다.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {        
        ListNode prev = null, current = head;

        while (current != null) {
            ListNode next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }

        return prev;
    }
}

 

방식2) recursive

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        return reverse(head, null);
    }

    public ListNode reverse(ListNode node, ListNode prev) {
        if (node == null) {
            return prev;
        }

        ListNode next = node.next;
        node.next = prev;
        return reverse(next, node);
    }
}