zoj3747（递推dp）

题目链接：http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemId=5170

题意：给n个士兵排队，每个士兵三种G、R、P可选，求至少有m个连续G士兵，最多有k个连续R士兵的排列的种数。

详细解法：http://blog.csdn.net/cc_again/article/details/24841249

注意下取模后为负的情况

#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <queue>
#include <cstdlib>
#include <stack>
#include <vector>
#include <set>
#include <map>
#define LL long long
#define mod 1000000007
#define inf 0x3f3f3f3f
#define N 1000010
#define clr(a) (memset(a,0,sizeof(a)))
using namespace std;
LL dp[N][];
LL m,n,k;
LL solve(LL u,LL v)
{
    dp[][]=;dp[][]=;dp[][]=;
    for(int i=;i<=n;i++)
    {
        LL sum=(dp[i-][]+dp[i-][]+dp[i-][])%mod;
        dp[i][]=sum;
        if(i<=u)dp[i][]=sum;
        else if(i==u+)dp[i][]=sum-;
        else dp[i][]=(sum-dp[i-u-][]-dp[i-u-][]+mod)%mod;
        if(i<=v)dp[i][]=sum;
        else if(i==v+)dp[i][]=sum-;
        else dp[i][]=(sum-dp[i-v-][]-dp[i-v-][]+mod)%mod;
    }
    return (dp[n][]+dp[n][]+dp[n][])%mod;
}
int main()
{
    while(scanf("%lld%lld%lld",&n,&m,&k)>)
    {
        printf("%lld\n",((solve(k,n)-solve(k,m-))%mod+mod)%mod);
    }
}