LeetCode-in-All

206. Reverse Linked List

Easy

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: []

Constraints:

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution

import com_github_leetcode.ListNode

/*
 * Example:
 * var li = ListNode(5)
 * var v = li.`val`
 * Definition for singly-linked list.
 * class ListNode(var `val`: Int) {
 *     var next: ListNode? = null
 * }
 */
class Solution {
    fun reverseList(head: ListNode?): ListNode? {
        var prev: ListNode? = null
        var curr = head
        while (curr !== null) {
            val next = curr.next
            curr.next = prev
            prev = curr
            curr = next
        }
        return prev
    }
}