206. Reverse Linked List

Reverse Linked List - LeetCode

 

주어진 단일 연결 리스트의 헤드가 주어졌을 때, 리스트를 역순으로 만들고 역순으로 된 리스트를 반환하세요.

 

 

Given the head of a singly linked list, reverse the list, and return the reversed list.

 

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

 

풀이방법

prev 변수는 역순으로 연결된 노드들의 첫 노드를 가리키고, current 변수는 현재 노드를 가리킵니다.
반복문을 통해 현재 노드의 next를 역순으로 연결하고, prev를 현재 노드로 이동합니다.
마지막에 prev를 반환하여 역순으로 연결된 리스트를 얻습니다.

 

 

/**
 * 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 beforeNode = null;

        while(head != null){
            ListNode nextNode = head.next;
            head.next = beforeNode;
            
            beforeNode = head;
            head = nextNode;
        }

        return beforeNode;
    }
}