題目鏈接：https://www.luogu.org/problemnew/show/P1494

#include <cstdio>
#include <algorithm>
#include <iostream>
#include <cmath>
using namespace std;
const int maxn =  50000+10;
inline long long read()
{
    long long k=0;
    char c;
    c=getchar();
    while(!isdigit(c))c=getchar();
    while(isdigit(c)){k=(k<<3)+(k<<1)+c-'0';c=getchar();}
    return k;
}
long long n, m, bl, answer, curL = 1, curR = 0, cnt[maxn], a[maxn], gg;

struct query{
    long long p, l, r;
}e[maxn];

struct answers{
    long long x, y;
}ans[maxn], now;

long long gcd(long long x, long long y)
{
    if(x%y == 0) return y;
    else return gcd(y,x%y);
}

bool cmp(query a, query b)
{
    return (a.l/bl) == (b.l/bl) ? a.r < b.r : a.l < b.l;
}

void add(long long pos)
{
    cnt[a[pos]]++;
    if(cnt[a[pos]] > 1) 
    now.x = now.x + cnt[a[pos]]*(cnt[a[pos]]-1)-(cnt[a[pos]]-1)*(cnt[a[pos]]-2);

}

void remove(long long pos)
{
    cnt[a[pos]]--;
    if(cnt[a[pos]] > 0) 
    now.x = now.x + cnt[a[pos]]*(cnt[a[pos]]-1)-(cnt[a[pos]]+1)*cnt[a[pos]];
}

int main()
{
    n = read(); m = read();
    
    bl = sqrt(n);
    
    for(long long i = 1; i <= n; i++)
    a[i] = read();
    
    for(long long i = 1; i <= m; i++)
    {
        e[i].l = read(); e[i].r = read(); e[i].p = i;
    }
    
    sort(e+1,e+1+m,cmp);
    
    for(long long i = 1; i <= m; i++)
    {
        long long L = e[i].l, R = e[i].r;
        while(curL < L) 
            remove(curL++);
        while(curL > L)
            add(--curL);
        while(curR < R)
            add(++curR);
        while(curR > R)
            remove(curR--);
        now.y = (e[i].r-e[i].l+1)*(e[i].r-e[i].l);
        if (!now.x) 
        {
            now.x=0;
            now.y=1;    
        }
        gg = gcd(now.x, now.y);
        ans[e[i].p].x = now.x/gg;
        ans[e[i].p].y = now.y/gg;
    }
    for(long long i = 1; i <= m; i++)
    printf("%lld/%lld\n",ans[i].x,ans[i].y);
    return 0;
}
 

【luogu P1494 [國家集訓隊]小Z的襪子】 題解