Swap Nodes in Pairs.cpp

// Runtime: 11 ms (Top 6.33%) | Memory: 7.5 MB (Top 92.11%)
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        // if head is NULL OR just having a single node, then no need to change anything
        if(head == NULL || head -> next == NULL)
        {
            return head;
        }

        ListNode* temp; // temporary pointer to store head -> next
        temp = head->next; // give temp what he want

        head->next = swapPairs(head->next->next); // changing links
        temp->next = head; // put temp -> next to head

        return temp; // now after changing links, temp act as our head
    }
};