树状数组专题(四)-数列-训练套题T10T3

树状数组专题(四)-数列-训练套题T10T3

代码如下：

#include 
#include 
#include 
#include 

using namespace std;

const int maxn = 50000+5;
const int maxm = 32771;
int a[maxn];
int c[maxn];
int tmp[maxn];
int n;
int lowbit(int x){
    return x & (-x);
}

void update(int x,int v){
    for(int i = x ; i < maxn ; i += lowbit(i)){
        c[i] += v;
    }
}

int getSum(int x){
    int sum = 0;
    for(int i = x ; i > 0 ; i -= lowbit(i)){
        sum += c[i];
    }
    return sum;
}

int main(){
    freopen("in.txt","r",stdin);
    while(scanf("%d",&n)!=EOF){
        long long ans = 0;
        memset(c,0,sizeof(c));
        for(int i = 1 ; i <= n ; i++){
            scanf("%d",&a[i]);
        }
        for(int i = 1 ; i <= n ; i++){
            update(a[i],1);
            tmp[i] = getSum(a[i]-1);
        }
        memset(c,0,sizeof(c));
        for(int i = n ; i >= 1 ; i--){
            update(a[i],1);
            ans += tmp[i]*getSum(a[i]-1);
        }
        printf("%lld",ans);
    }


    return 0;
}

运行结果：