poj2299 Ultra-QuickSort

逆序对模板题，过水不解释

// poj 2299
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 500010;
int a[N],x[N];
struct TreeArray {
    int t[N];
    int lowbit(int a) {
        return a & (-a);
    }
    void add(int x, int a) {
        if(a == 0 || x == 0) {
            return;
        }
        for(int i = x; i < N; i += lowbit(i)) {
            t[i] += a;
        }
        return;
    }
    int getsum(int x) {
        if(x == 0) {
            return 0;
        }
        int ans = 0;
        for(int i = x; i > 0; i -= lowbit(i)) {
            ans += t[i];
        }
        return ans;
    }
    int ask(int l, int r) {
        return getsum(r) - getsum(l - 1);
    }
    void clear() {
        memset(t,0,sizeof(t));
        return;
    }
}ta;

int main() {
    int n;
    scanf("%d", &n);
    x[0] = -1;
    while(n) {
        ta.clear();
        for(int i = 1; i <= n; i++) {
            scanf("%d", &a[i]);
            x[i] = a[i];
        }
        sort(x + 1, x + n + 1);
        int k = 0;
        for(int i = 1; i <= n; i++) {
            if(x[i] != x[i-1]) {
                x[++k] = x[i];
            }
        }
        /// /////////43
        long long ans = 0;
        for(int i = n; i > 0; i--) {
            int p = lower_bound(x + 1, x + k + 1, a[i]) - x;
            ans += ta.getsum(p - 1);
            ta.add(p, 1);
        }
        printf("%I64d\n", ans);
        scanf("%d", &n);
    }
    return 0;
}