我写了个莫队，看起来好像没什么问题，但是 WA 了。

瞪了半天没瞪出来，也没有小数据。

#include<bits/stdc++.h>
#define ll long long
using namespace std;
int n, m;
struct que{
	int l, r, id;
	ll fz, fm; 
}qu[100005];
int c[100005], kuai[1005], buk[100005];
ll ans;
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	cin >> n >> m;
	for (int i = 1; i <= n; i++)
		cin >> c[i];
	for (int i = 1; i <= m; i++){
		cin >> qu[i].l >> qu[i].r;
		qu[i].id = i;
	}
	sort(qu + 1, qu + 1 + m, [](que a, que b){return a.l < b.l;});
	int k1 = sqrt(m), k2 = ceil(m * 1.00 / k1) + 0.114514;
	for (int i = 1; i <= k2; i++)
		kuai[i] = k1 * (i - 1) + 1;
	kuai[k2 + 1] = m + 1;
	for (int i = 1; i <= k2; i++)
		sort (qu + kuai[i], qu + kuai[i + 1], [](que a, que b){return a.r < b.r;});
	for (int i = 1; i <= k2; i++){
		memset(buk, 0, sizeof buk);
		ans = 0;
		for (int j = qu[kuai[i]].l; j <= qu[kuai[i]].r; j++)
			buk[c[j]]++, ans += buk[c[j]] - 1;
		ll len = qu[kuai[i]].r - qu[kuai[i]].l + 1;
		ll nans = ans;
		ll fm = len * (len - 1) / 2;
		ll g = __gcd(fm, nans);
		if (qu[kuai[i]].l != qu[kuai[i]].r){
			qu[kuai[i]].fz = nans / g;
			qu[kuai[i]].fm = fm / g;
		}else{
			qu[kuai[i]].fz = 0;
			qu[kuai[i]].fm = 1;	
		}
		for (int ii = kuai[i] + 1; ii < kuai[i + 1]; ii++){
			for (int j = qu[ii - 1].r + 1; j <= qu[ii].r; j++)
				buk[c[j]]++, ans += buk[c[j]] - 1;
			if (qu[ii].l > qu[ii - 1].l){
				for (int j = qu[ii - 1].l; j < qu[ii].l; j++)
					buk[c[j]]--, ans += buk[c[j]];
			}else{
				for (int j = qu[ii].l; j < qu[ii - 1].l; j++)
					buk[c[j]]++, ans += buk[c[j]] - 1;
			}
			ll len = qu[ii].r - qu[ii].l + 1;
			ll nans = ans;
			ll fm = len * (len - 1) / 2;
			ll g = __gcd(fm, nans);
			if (qu[ii].l != qu[ii].r){
				qu[ii].fz = nans / g;
				qu[ii].fm = fm / g;
			}else{
				qu[ii].fz = 0;
				qu[ii].fm = 1;	
			}
		}
	}
	sort (qu + 1, qu + 1 + m, [](que a, que b){return a.id < b.id;});
	for (int i = 1; i <= m; i++)
		cout << qu[i].fz << "/" << qu[i].fm << "\n"; 
}