Codeforces 258D Little Elephant and Broken Sorting (逆序对，概率)

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

#define N 1020

int a[N];
double p[N][N];
int n, m;


int main() {
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; ++i) scanf("%d", &a[i]);
	for(int i = 1; i <= n; ++i) {
		for(int j = 1; j <= n; ++j) {
			p[i][j] = a[i] > a[j]? 1: 0;
		}
	}
	for(int i = 1; i <= m; ++i) {
		int u, v;
		scanf("%d%d", &u, &v);
		for(int j = 1; j <= n; ++j) {
			if(j == u || j == v) continue;
			 p[j][u] = p[j][v] = (p[j][u] + p[j][v]) / 2;
			 p[u][j] = p[v][j] = (p[u][j] + p[v][j]) / 2;
		}
		p[u][v] = p[v][u] = (p[u][v] + p[v][u]) / 2;
	}
	double ans = 0;
	for(int i = 1; i <= n; ++i) {
		for(int j = i + 1; j <= n; ++j)
			ans += p[i][j];
	}
	printf("%.12lf\n", ans);
	return 0;
}