Linked List

1// Iterative reversal — O(N) time, O(1) space
2ListNode* reverseList(ListNode* head)
3{
4    ListNode* prev = nullptr;
5    ListNode* curr = head;
6 
7    while (curr)
8    {
9        ListNode* next = curr->next;   // save before overwriting
10 
11        curr->next = prev;             // flip pointer
12        prev       = curr;             // advance prev
13        curr       = next;             // advance curr
14    }
15 
16    return prev;   // new head
17}