我当时的思路和编码：map

#include<iostream>
#include<vector>
#include<map>
using namespace std;
int main()
{

	int num;
	int a;
	map<int, int>mp;
	cin >> num;
	vector<int>input(num);
	for (int i = 0; i < num; i++)
	{
		cin >> input[i];
		mp[input[i]]++;
	}
	int max_count = 1;
	int first_number = -1;
	int second_number = -1;
	for (auto it = mp.begin(); it != mp.end(); it++)
	{
		if (it->second > max_count)
		{
			max_count = it->second;
			first_number = it->first;
		}
	}
	if (max_count >= 4)
	{
		cout << first_number*first_number << endl;
	}
	else if (max_count == 1)
		cout << -1 << endl;
	else
	{
		mp.erase(first_number);
		max_count = 1;

		for (auto it = mp.begin(); it != mp.end(); it++)
		{
			if (it->second > max_count)
			{
				max_count = it->second;
				second_number = it->first;
			}
		}
		if (max_count == 1)cout << -1 << endl;
		else
			cout << first_number*second_number << endl;
	}



	system("pause");
	return 0;
}

我的思路和编码：

#include<iostream>
#include<vector>
using namespace std;
int main()
{
	int num;
	cin >> num;
	vector<int>input(num + 1, 0);
	for (int i = 1; i <= num; i++)
	{
		cin >> input[i];
	}
	int res = 0;
	for (int i = 0; i <= num; i++)
	{
		if (i != input[i])
		{
			swap(input[i], input[input[i]]);
			res++;
		}
	}
	cout << res << endl;
	system("pause");
	return 0;
}

第三题不知道怎么做，感觉应该要用贪心算法。