programming-examples/java/Data_Structures/MosAlgorithm.java

import java.util.Arrays;

// https://www.hackerearth.com/notes/mos-algorithm/
// Solution of http://www.spoj.com/problems/DQUERY/en/
public class MosAlgorithm {

	public static class Query {
		int index;
		int a;
		int b;

		public Query(int a, int b) {
			this.a = a;
			this.b = b;
		}
	}

	static int add(int[] a, int[] cnt, int i) {
		return ++cnt[a[i]] == 1 ? 1 : 0;
	}

	static int remove(int[] a, int[] cnt, int i) {
		return --cnt[a[i]] == 0 ? -1 : 0;
	}

	public static int[] processQueries(int[] a, Query[] queries) {
		for (int i = 0; i < queries.length; i++) queries[i].index = i;
		int sqrtn = (int) Math.sqrt(a.length);
		Arrays.sort(queries, (q1, q2) -> {
			int cmp = Integer.compare(q1.a / sqrtn, q2.a / sqrtn);
			return cmp != 0 ? cmp : Integer.compare(q1.b, q2.b);
		});
		int[] cnt = new int[1000_002];
		int[] res = new int[queries.length];
		int L = 1;
		int R = 0;
		int cur = 0;
		for (Query query : queries) {
			while (L < query.a) cur += remove(a, cnt, L++);
			while (L > query.a) cur += add(a, cnt, --L);
			while (R < query.b) cur += add(a, cnt, ++R);
			while (R > query.b) cur += remove(a, cnt, R--);
			res[query.index] = cur;
		}
		return res;
	}

	public static void main(String[] args) {
		int[] a = {1, 3, 3, 4};
		Query[] queries = {new Query(0, 3), new Query(1, 3), new Query(2, 3), new Query(3, 3)};
		int[] res = processQueries(a, queries);
		System.out.println(Arrays.toString(res));
	}
}