206. Reverse Linked List_iterative.cc

//using iterative method
//TC: O(n), where n is the number of nodes
//SC: O(1)
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = nullptr;
        while (head != NULL) {
            ListNode* next = head->next;
            head->next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
};