题解 | 谐距下标对

#include <iostream>
#include <map>
#include <vector>
using namespace std;

int main() {
    int n;
    cin>>n;
    vector<int> arr(n,0);
    for(int i=0;i<n;i++)
    cin>>arr[i];
    vector<int> B(n,0);
    for(int i=0;i<n;i++){
        B[i]=arr[i]-i;
    }
    map<int, int> mymap;
    for(int i=0;i<n;i++){
        if(mymap.find(B[i]) != mymap.end()){
            mymap[B[i]]=mymap[B[i]]+1;
        }else{
            mymap[B[i]]=1;
        }
    }
    long long res=0;
    for(const auto& [key,value]:mymap){
        long long cnt=(long long)value*(value-1)/2;
        res+=cnt;
    }
    cout<<res<<endl;
    return 0;
}
// 64 位输出请用 printf("%lld")