【蓝桥备赛】wzy的数组Ⅱ——简单莫队问题

题目链接

wzy的数组Ⅱ

个人思路

本题需要统计区间范围内 数值为 x 在区间出现次数也为 x 的数的个数。区间询问 + 多次询问，我们选择 莫队。
将多次询问按照区间边界进行排序，每一次区间的移动，先去判断当前区间指针所指向的数是否符合题目条件，然后对该数的数量进行对应的增减操作，操作完之后，仍需判断当前数是否符合题目条件，因为数量发生了变化。

参考代码

C++

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e5 + 3;/*
https://www.lanqiao.cn/problems/3247/learning/
*/int n, m, maxn, arr[N], cnt[N], sumn = 0, ans[N];class Query
{
public:int id, l, r;int operator<(const Query &x) const{if (l / maxn != x.l / maxn)return l < x.l;return (l / maxn) & 1 ? r < x.r : r > x.r;}
} q[N];void add(int i)
{if (cnt[arr[i]] == arr[i])sumn--;cnt[arr[i]]++;if (cnt[arr[i]] == arr[i])sumn++;
}void del(int i)
{if (cnt[arr[i]] == arr[i])sumn--;cnt[arr[i]]--;if (cnt[arr[i]] == arr[i])sumn++;
}int main()
{cin >> n >> m;maxn = sqrt(n);for (int i = 1; i <= n; ++i)cin >> arr[i];for (int i = 0; i < m; ++i){q[i].id = i;cin >> q[i].l >> q[i].r;}sort(q, q + m);int l = 1, r = 0;for (int i = 0; i < m; ++i){while (l > q[i].l){add(--l);}while (r < q[i].r){add(++r);}while (l < q[i].l){del(l++);}while (r > q[i].r){del(r--);}ans[q[i].id] = sumn;}for (int i = 0; i < m; ++i){cout << ans[i] << "\n";}return 0;
}

Java

import java.util.Arrays;
import java.util.Scanner;public class Main {static class Query implements Comparable<Query> {public int id, l, r;@Overridepublic int compareTo(Query x) {if (l / maxn != x.l / maxn)return Integer.compare(l, x.l);return (l / maxn) % 2 == 1 ? Integer.compare(r, x.r) : Integer.compare(x.r, r);}}static int n, m, maxn, sumn = 0;static int[] arr, cnt, ans;static Query[] q;static void add(int i) {if (cnt[arr[i]] == arr[i])sumn--;cnt[arr[i]]++;if (cnt[arr[i]] == arr[i])sumn++;}static void del(int i) {if (cnt[arr[i]] == arr[i])sumn--;cnt[arr[i]]--;if (cnt[arr[i]] == arr[i])sumn++;}public static void main(String[] args) {Scanner scanner = new Scanner(System.in);n = scanner.nextInt();m = scanner.nextInt();maxn = (int) Math.sqrt(n);arr = new int[n + 1];cnt = new int[n + 1];ans = new int[m];q = new Query[m];for (int i = 1; i <= n; ++i)arr[i] = scanner.nextInt();for (int i = 0; i < m; ++i) {q[i] = new Query();q[i].id = i;q[i].l = scanner.nextInt();q[i].r = scanner.nextInt();}Arrays.sort(q, 0, m);int l = 1, r = 0;for (int i = 0; i < m; ++i) {while (l > q[i].l) {add(--l);}while (r < q[i].r) {add(++r);}while (l < q[i].l) {del(l++);}while (r > q[i].r) {del(r--);}ans[q[i].id] = sumn;}for (int i = 0; i < m; ++i) {System.out.println(ans[i]);}}
}

由于还处于初学莫队，找了几个简单的莫队类型题目练练手，近期类似问题做了好几个，有兴趣的可以去我的蓝桥专栏下面看看。