水题 * 3 。

#include <bits/stdc++.h>
using namespace std;

int main() {
	int n;
	map<int, int> ans; 
	cin >> n;
	for(int front = 1; front <= n; front++){
		int contrary;
		cin >> contrary;
		// 计算并存入数组
		ans[abs(front - contrary)]++; 
	}
	for(auto a = ans.rbegin(); a != ans.rend(); a++)
		if(a->second > 1) 
			cout << a->first << " " << a->second << endl;
}