51nod 1020 逆序排列

1020 逆序排列

#include <algorithm>
#include <cstring>
#include <cstdio>
using namespace std;
#define INF 0x3fffffff
typedef long long ll;
const int P=1e9+7;
const int N=20005;
int ans
,dp[3]
;
struct node
{
int n,k;
int id;
}b
;
bool cmp(node n1,node n2)
{
if (n1.n==n2.n) return n1.k < n2.k;
else return n1.n < n2.n;
}
int n,mk;
int main()
{
register int i,j,k,t;
scanf("%d",&t);
n=0;mk=0;
for (i=1;i<=t;i++)
{
scanf("%d%d",&b[i].n,&b[i].k);
b[i].id=i;
n=max(n,b[i].n);
mk=max(mk,b[i].k);
}
memset(ans,0,sizeof(ans));
sort(b+1,b+t+1,cmp);
int st,prs,pre=0,now=1,p=1;dp[0][0]=1;
for (i=2;i<=n;i++)
{
k=min(i*(i-1)/2,mk);
for (j=0;j<=k;j++)
{
st=max(0,j-i+1);
prs=st==0?0:dp[pre][st-1];
dp[now][j]=(dp[pre][j]-prs+P)%P;
if (j>=1)dp[now][j]=(dp[now][j]+dp[now][j-1])%P;
}
while (p<=t)
{
if (b[p].n!=i) break;
ans[b[p].id]=dp[now][b[p].k];
p++;
}
now^=1;pre^=1;
}
for (i=1;i<=t;i++) printf("%d\n",ans[i]);
return 0;
}