#include <iostream>

/**
 * Definition for singly-linked list.
 */
struct ListNode {
	int val;
	ListNode* next;

	explicit ListNode(int x = 0, ListNode* next = nullptr) : val(x), next(next) {}
};

ListNode* construct(int x) {
	return new ListNode(x);
}

template<typename... Ts>
ListNode* construct(int x, Ts... xs) {
	return new ListNode(x, construct(xs...));
}

class Solution {
public:
	static ListNode* swapPairs(ListNode* head) {
		ListNode ret(0, head);
		auto* ptr = &ret;
		while (ptr) {
			ListNode* ptrN = ptr->next;
			if (!ptrN)
				break;

			ListNode* ptrNN = ptrN->next;
			if (!ptrNN)
				break;

			// ptr --> ptrN --> ptrNN --> ptrNNN
			// ListNode* ptrNNN = ptrNN->next;

			// ptr is previous node
			// swap ptrN and ptrNN.
			(ptr = ((ptr->next = ptrNN)->next = ptrN))->next = ptrNN->next;

			// ptr --> ptrNN --> ptrN --> ptrNNN
			// ptr = ptrN;
		}
		return ret.next;
	}
};

int main() {
	auto* head = construct(1, 2, 3, 4, 5, 6);
	head = Solution::swapPairs(head);
	for (auto* ptr = head; ptr; ptr = ptr->next) {
		std::printf("%d ", ptr->val);
	}
	return 0;
}