24. Swap Nodes in Pairs.txt

//Îïòèìàëüíûé êîä.

ListNode* swapPairs(ListNode* head) {
	ListNode dummy(0, head);  //Ñîçäà¸ì -1 óçåë
	ListNode *pre = &dummy, *first = head;//Äîáàâëÿåì 2 óêàçàòåëÿ íà -1 è 1 óçåë

	while (first && first->next) {//Ïîêà 1ûé è 2îé óçåë ñóùåñòâóþò
		ListNode *second = first->next;//Èíèöèàëèçàöèÿ âòîðîãî óçëà
		ListNode *nextPair = second->next;//Èíèöèàëèçàöèÿ ñëåäóþùåé ïàðû

		pre->next = second;//next -1îãî óçëà ìåíÿåì íà 2îé óçåë âìåñòî 1îãî 
		first->next = nextPair;//next 1îãî óçëà ìåíÿåì íà 3èé óçåë âìåñòî 2îãî
		second->next = first;//next âòîðîãî óçëà ìåíÿåì íà 1ûé âìåñòî 3åãî

		pre = first;//Ïîäãîòîâêà ê ñëåäóþùåé èòåðàöèè. pre òåïåðü 2îé óçåë
		first = nextPair;//Ïîäãîòîâêà ê ñëåäóþùåé èòåðàöèè. first òåïåðü 3èé óçåë
	}
	return dummy.next;//Âîçâðàò 1 óçëà
}





//Ìîé êîä. Ïðîñòî ïîìåíÿòü óêàçàòåëè ìåñòàìè è âñå. 
/**
 * 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* swapPairs(ListNode* head) {
        if(head == NULL)
            return {};
        if(head->next == NULL)
            return head;
        ListNode *previousNode = head;
	    ListNode *secondNude = previousNode->next;
	    ListNode *answer = secondNude;
        ListNode *lastNode = previousNode;
        while (previousNode != NULL && previousNode->next != NULL) {
            secondNude = previousNode->next;
            ListNode *temp = secondNude->next;
            secondNude->next = previousNode;
            if (previousNode != head)
                lastNode->next = secondNude;
            previousNode->next = temp;
            lastNode = previousNode;
            previousNode = temp;
        }
        return answer;
    }
};