UVALive 6801 Sequence (DP)

dp[i][j] : 还有 i 个1，进行了 j 次翻转操作的方法数。

Problem

Source : Regionals 2014 >> Asia - Kuala Lumpur

#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;
#define prt(k) cout<<#k" = "<<k<<endl;

const int N = 2333;
typedef long long ll;
const ll mod = 1e9+7;
ll dp[N][2];
char s[N];
int n, m;
void add(ll &a, ll b) { a=(a+b)%mod; }
int main()
{
    int re; cin>>re;
    int ca = 1;
    while (re--)
    {
        cin>>n>>m;
        int cnt = 0;
        for (int i=0;i<n;i++) {
            int x; scanf("%d", &x);
            if(x==1) cnt++;
        }
        int now = 0;
        memset(dp ,0, sizeof dp);
        dp[cnt][now] = 1;
        for (int j=0;j<m;j++)
        {
            for(int i=n;i>=0;i--)
            {
                if(i>0)
                {
                    add(dp[i-1][1-now], dp[i][now] * i % mod);
                }
                add(dp[i+1][1-now], dp[i][now] * (n-i) % mod);
            }
            for(int i=0;i<=n;i++) dp[i][now] = 0;
            now=1-now;
        }
        printf("Case #%d: %lld\n",ca++, dp[0][now]);
    }
}