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

# Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end
# @param {ListNode} head
# @return {ListNode}
def reverse_list(head)
  prev = nil
  curr = head
  while curr
    next_node = curr.next
    curr.next = prev
    prev = curr
    curr = next_node
  end
  prev
end