「国家集训队」小Z的袜子

「国家集训队」小Z的袜子

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莫队板子题。
注意计算答案的时候，由于分子分母都要除以2，所以可以直接约掉，这样在开桶算的时候也方便一些。
参考代码：

#include <algorithm>
#include <cstdio>
#include <cmath>
#define rg register
#define file(x) freopen(x".in", "r", stdin), freopen(x".out", "w", stdout)
#define int long long 
using namespace std;
template < class T > inline void read(T& s) {
    s = 0; int f = 0; char c = getchar();
    while ('0' > c || c > '9') f |= c == '-', c = getchar();
    while ('0' <= c && c <= '9') s = s * 10 + c - 48, c = getchar();
    s = f ? -s : s;
}

const int _ = 50010;

int n, gap, m, c[_], pos[_], s[_], ans;

struct Ask { int l, r, id; } ask[_];
struct Ans { int a, b; } res[_];
inline bool cmp(const Ask& x, const Ask& y)
{ return pos[x.l] == pos[y.l] ? x.r < y.r : x.l < y.l; }

inline void update(int x, int v)
{ ans -= s[c[x]] * s[c[x]], s[c[x]] += v, ans += s[c[x]] * s[c[x]]; }

signed main() {
#ifndef ONLINE_JUDGE
    file("cpp");
#endif
    read(n), read(m), gap = sqrt(1.0 * n);
    for (rg int i = 1; i <= n; ++i)
        read(c[i]), pos[i] = (i - 1) / gap + 1;
    for (rg int i = 1; i <= m; ++i)
        read(ask[i].l), read(ask[i].r), ask[i].id = i;
    sort(ask + 1, ask + 1 + m, cmp);
    for (rg int i = 1, l = 1, r = 0; i <= m; ++i) {
        while (l < ask[i].l) update(l++, -1);
        while (l > ask[i].l) update(--l, 1);
        while (r < ask[i].r) update(++r, 1);
        while (r > ask[i].r) update(r--, -1);
        int now = ask[i].id;
        if (ask[i].l == ask[i].r) { res[now].a = 0, res[now].b = 1; continue; }
        res[now].a = ans - (ask[i].r - ask[i].l + 1);
        res[now].b = 1ll * (ask[i].r - ask[i].l + 1) * (ask[i].r - ask[i].l);
        int g = __gcd(res[now].a, res[now].b);
        res[now].a /= g, res[now].b /= g;
    }
    for (rg int i = 1; i <= m; ++i) printf("%lld/%lld\n", res[i].a, res[i].b);
    return 0;
}