206. Reverse Linked List

1. Description

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

2. Example

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: []

3. Constraints

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

4. Solutions

Double Pointers

n is the number of nodes in head
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    ListNode *reverseList(ListNode *head) {
        ListNode *prev_node = nullptr, *current_node = head;
        while (current_node != nullptr) {
            ListNode *temp = current_node->next;
            current_node->next = prev_node;

            prev_node = current_node;
            current_node = temp;
        }

        return prev_node;
    }
};