题解 | 谐距下标对

import java.util.Scanner;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.StreamTokenizer;
import java.util.HashMap;
import java.util.Map;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StreamTokenizer in = new StreamTokenizer(br);

        PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));

        in.nextToken();
        int n = (int)in.nval;

        Map<Integer, Integer> hMap = new HashMap<>();

        for (int i = 0; i < n; i++) {
            in.nextToken();
            int a = (int)in.nval;

            int diff = a - i;

            hMap.put(diff, hMap.getOrDefault(diff, 0) + 1);
        }

        long total = 0;

        for (int count : hMap.values()) {
            if (count > 1) {
                total += (long)count * (count - 1) / 2;
            }
        }

        out.println(total);

        out.flush();
        out.close();
        br.close();
    }
}