SGU 180.Inversions

没什么好说的离散化求逆序对，树状数组即可

 

code

#include <iostream>
#include <algorithm>
#define INF 66666
using namespace std;
int A[INF], g[INF], pos[INF], n;
long long ans;
void update (int x, int k) {
	for (; x <= n; x += x & -x)  A[x] += k;
}
long long getsum (int x) {
	long long  sum = 0;
	for (; x > 0; x -= -x & x)   sum += A[x];
	return sum;
}
bool cmp (int a, int b) {
	return g[a] < g[b];
}
int  main() {
	cin >> n;
	for (int i = 1; i <= n; i++) cin >> g[i], pos[i] = i;
	stable_sort (pos + 1, pos + 1 + n, cmp);
	for (int i = 1; i <= n; i++) g[pos[i]] = i;
	for (int i = n; i >= 1; i--) {
		update (g[i], 1);
		ans += getsum (g[i] - 1);
	}
	cout << ans << endl;
}