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

/*
 * Definition for singly-linked list.
 * class ListNode(_x: Int = 0, _next: ListNode = null) {
 *   var next: ListNode = _next
 *   var x: Int = _x
 * }
 */
object Solution {
    def 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
        }
        prev
    }
}