51nod1779 逆序对统计

#include <bits/stdc++.h>
using namespace std;
const int N = 105, M = (1<<20) + 5;
int n, m, a[N], all, ff[M], f[2][M], cur;
int main() {
    scanf("%d%d",&n,&m);
    for(int i=1; i<=m; i++){
    int p;
    scanf("%d",&p);
    a[i]=p-1;
    }
    all = 1<<n;
    for(int i=0; i<=n; i++) ff[1<<i] = 1;
    for(int i=1; i<all; i++) ff[i] = ff[i&-i] + ff[i^(i&-i)];
    memset(f, -1, sizeof(f));
    f[cur][0] = 0;
    for(int i=0; i<m; i++, cur ^= 1) {
        int *g = f[cur], *d = f[cur^1];
        for(int s=0; s<all; s++) if(g[s] != -1) {
            d[s] = max(d[s], g[s]);
            if(~ s & (1<<a[i+1])) {
                int ns = s | (1<<a[i+1]);
                d[ns] = max(d[ns], g[s] + ff[ns >> (a[i+1] + 1)]);
            }
            g[s] = -1;
        }
    }
    printf("%d\n", f[cur][all-1]);
    return 0;
}