莫队 - 小Z的袜子(hose)

题目链接：https://www.lydsy.com/JudgeOnline/problem.php?id=2038

取袜子总的方案数为(r−l+1)∗(r−l)/2，取到两只同颜色的袜子的方案数sum[i]∗(sum[i]−1)/2的累加，sum[i]为每种颜色袜子的数量。
套下莫队的模板就好。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>

using namespace std;

typedef long long ll;
ll n, m, block, te;
const int maxn = 5e5+5;
int a[maxn], num[maxn];
int ans[maxn][2];
struct node{
    ll l, r, id;
}q[maxn];

ll gcd(ll a, ll b)
{
    while(b > 0)
    {
        ll tmp = b;
        b = a%b;
        a = tmp;
    }
    return a;
}

ll C(ll x)
{
    return x*(x-1)/2;
}

bool cmp(const node &s, const node &s1)
{
    if(s.l/block == s1.l/block) return s.r < s1.r;
    return s.l < s1.l;
}

void add(int x)
{
    te -= C(num[a[x]]);
    num[a[x]]++;
    te += C(num[a[x]]);
}

void del(int x)
{
    te -= C(num[a[x]]);
    num[a[x]]--;
    te += C(num[a[x]]);
}

int main()
{
    ios::sync_with_stdio(false);
    while(cin >> n >> m)
    {
        memset(num, 0, sizeof num);
        block = ceil(sqrt(n));
        for(int i = 1; i <= n; i++)
            cin >> a[i];
        for(int i = 1; i <= m; i++)
        {
            cin >> q[i].l >> q[i].r;
            q[i].id = i;
        }
        sort(q+1, q+1+m, cmp);
        int l = 1, r = 0;
        te = 0;
        for(int i = 1; i <= m; i++)
        {
            while(l < q[i].l) del(l++);
            while(l > q[i].l) add(--l);
            while(r < q[i].r) add(++r);
            while(r > q[i].r) del(r--);
        
            ll  t = C(q[i].r-q[i].l+1);
            ll  g = gcd(t, te);
            
            ans[q[i].id][0] = te/g;
            ans[q[i].id][1] = t/g;
        }
        for(int i = 1; i <= m; i++)
            printf("%d/%d\n", ans[i][0], ans[i][1]);
    }
    return 0;    
}