归并排序

直接在归并排序改一个地方就行了，就是a[x]>a[y]的时候，多一步计数即可。
也可以采取树状数组查询之前比你大的数出现几个，数据较小可以不离散处理，大一点可以离散。

#pragma GCC target("avx,sse2,sse3,sse4,popcnt")
#pragma GCC optimize("O2,O3,Ofast,inline,unroll-all-loops,-ffast-math")
#include <bits/stdc++.h>
using namespace std;
#define js ios::sync_with_stdio(false);cin.tie(0); cout.tie(0)
typedef long long ll; typedef unsigned long long ull; typedef long double ld;
const ll MOD = 1e9 + 7;
inline ll read() { ll s = 0, w = 1; char ch = getchar(); while (ch < 48 || ch > 57) { if (ch == &#39;-&#39;) w = -1; ch = getchar(); }    while (ch >= 48 && ch <= 57) s = (s << 1) + (s << 3) + (ch ^ 48), ch = getchar();    return s * w; }
inline void write(ll x) { if (!x) { putchar(&#39;0&#39;); return; } char F[40]; ll tmp = x > 0 ? x : -x; if (x < 0)putchar(&#39;-&#39;);    int cnt = 0;    while (tmp > 0) { F[cnt++] = tmp % 10 + &#39;0&#39;;        tmp /= 10; }    while (cnt > 0)putchar(F[--cnt]); }
inline ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
ll qpow(ll a, ll b) { ll ans = 1;    while (b) { if (b & 1)    ans *= a;        b >>= 1;        a *= a; }    return ans; }    ll qpow(ll a, ll b, ll mod) { ll ans = 1; while (b) { if (b & 1)(ans *= a) %= mod; b >>= 1; (a *= a) %= mod; }return ans % mod; }
inline int lowbit(int x) { return x & (-x); }

const int N = 1e5 + 7;
int a[N], b[N];
ll ans = 0;

void merge_sort(int l, int r) {
    if (l == r)    return;
    int mid = l + r >> 1;
    merge_sort(l, mid);
    merge_sort(mid + 1, r);
    int x = l, y = mid + 1, i = 0;
    for (; i < r - l + 1; ++i) {
        if (x <= mid && y <= r) {
            if (a[x] > a[y])
                ans += mid - x + 1, b[i] = a[y++];
            else    b[i] = a[x++];
        }
        else {
            if (x <= mid)    b[i] = a[x++];
            if (y <= r)    b[i] = a[y++];
        }
    }
    while (~(--i))    a[r--] = b[i];
}

int main() {
    int n = read();
    for (int i = 1; i <= n; ++i)    a[i] = read();
    merge_sort(1, n);
    write(ans), putchar(10);
    return 0;
}