洛谷 P2709 小B的询问

题目链接

https://www.luogu.org/problem/P2709

分析

莫队算法，每次区间调整根据次数变化前后的平方差更新答案

AC代码

#include 
#include 
#include 

using namespace std;

inline int get_num() {
    int num = 0;
    char c = getchar();
    while (c < '0' || c > '9') c = getchar();
    while (c >= '0' && c <= '9')
        num = num * 10 + c - '0', c = getchar();
    return num;
}

const int maxn = 5e4 + 5;

struct Query {
    int l, r, id, b;
    bool operator < (const Query& rhs) const {
        if (b == rhs.b) return r < rhs.r;
        return b < rhs.b;
    }
} q[maxn];

int a[maxn], c[maxn], ans[maxn];

int main() {
    int n = get_num(), m = get_num(), k = get_num();
    for (int i = 1; i <= n; ++i) a[i] = get_num();
    int s = sqrt(n);
    for (int i = 1; i <= m; ++i) {
        q[i].l = get_num(), q[i].r = get_num();
        q[i].id = i;
        q[i].b = q[i].l / s;
    }
    sort(q + 1, q + m + 1);
    int l = 1, r = 0, Ans = 0;
    for (int i = 1; i <= m; ++i) {
        while (l > q[i].l) ++c[a[--l]], Ans += 2 * c[a[l]] - 1;
        while (r < q[i].r) ++c[a[++r]], Ans += 2 * c[a[r]] - 1;
        while (l < q[i].l) --c[a[l]], Ans -= 2 * c[a[l++]] + 1;
        while (r > q[i].r) --c[a[r]], Ans -= 2 * c[a[r--]] + 1;
        ans[q[i].id] = Ans;
    }
    for (int i = 1; i <= m; ++i) printf("%d\n", ans[i]);
    return 0;
}