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:
- The number of nodes in the list is the range
[0, 5000]. -5000 <= Node.val <= 5000
Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?
Solution
; Definition for singly-linked list:
#|
; val : integer?
; next : (or/c list-node? #f)
(struct list-node
(val next) #:mutable #:transparent)
; constructor
(define (make-list-node [val 0])
(list-node val #f))
|#
(define (reverse-list head)
(define (rev ll prev)
(match ll
[#f prev]
[(list-node val next) (rev next (list-node val prev))]))
(rev head #f))