Reverse Linked List

Iteration:

struct ListNode* reverseList(struct ListNode* head) {
    if (!head) return NULL;
    
    struct ListNode *res = (struct ListNode *)malloc(sizeof(struct ListNode));
    struct ListNode *cur = head;
    struct ListNode *next = cur->next;
    
    res->next = head;
    while (next) {
        cur->next = next->next;
        next->next = res->next;
        res->next = next;
        next = cur->next;
    }
    
    return res->next;
}

Recursion:

class Solution {
public:
    void recursion(ListNode** head) {
        if (!head) return;
        ListNode *cur = *head;
        ListNode *next = cur->next;
        if (!next) return;
        recursion(&next);
        cur->next->next = cur;
        cur->next = NULL;
        *head = next;
    }
    ListNode* reverseList(ListNode* head) {
        if(!head) return NULL;
        recursion(&head);
        return head;
    }
};