python/TangDou/AcWing/BIT/241_BIT.cpp

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
const int N = 200010;

int n, a[N];         // n个元素，a[i]代表原始值
LL big[N], small[N]; // big[i]:比a[i]大的元素个数，small[i]:比a[i]小的元素个数
LL res1, res2;       // V的个数，∧的个数

// 树状数组模板
typedef long long LL;
#define lowbit(x) (x & -x)
int c[N];
void add(int x, int d) {
    for (int i = x; i < N; i += lowbit(i)) c[i] += d;
}
LL sum(int x) {
    LL res = 0;
    for (int i = x; i; i -= lowbit(i)) res += c[i];
    return res;
}

// 桶计数+前缀和
int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) scanf("%d", &a[i]);

    for (int i = 1; i <= n; i++) { // 枚举每个点，由左到右一个个下场
        int x = a[i];
        small[i] = sum(x - 1);        // 在我前面出场，并且，值比我小, 用于方便统计铁锹(∧)
        big[i] = sum(n) - sum(x - 1); // 在我前面出场，并且，值比我大, 用于方便统计尖刀(V)
        add(x, 1);                    // 以值为桶的标识，记录此桶中数量+1
    }

    memset(c, 0, sizeof c);

    for (int i = n; i; i--) {
        int x = a[i];
        res1 += small[i] * sum(x - 1);
        res2 += big[i] * (sum(n) - sum(x - 1));
        add(x, 1);
    }
    printf("%lld %lld\n", res2, res1);
    return 0;
}