51nod 1020 逆序排列 递推DP

#include<cstdio>
using namespace std;

typedef long long ll;
const int MAXK = 2e4+5;
const int MAXN = 1e3+5;
const int mod = 1e9+7;
#define min(a,b) (a<b)?a:b

int n,k,dp[MAXN][MAXK];

// dp[n,k] = dp[n,k-1] + dp[n-1,k] - dp[n-1,k-n];

int getMod(ll t) {
    if(t >= mod) return t-mod;
    if(t<0) return t+mod;
    return t;
}

void init() {
    int i,j;
    for(i=2;i<=1000;i++) {
        dp[i][0]=1;
        for(j=1;j<=i*(i-1)/2&&j<=20000;j++) {
            ll tmp=0;
            ll tmp1=dp[i][j-1];
            ll tmp2=dp[i-1][j];
            ll tmp3=(j>=i)?dp[i-1][j-i]:0;
            tmp = tmp1+tmp2-tmp3;
            dp[i][j] = getMod(tmp);
        }
    }
}
int main () {
    init();
    int T; scanf("%d",&T);
    while (T--){
        scanf("%d %d",&n,&k);
        printf("%d\n",dp[n][k]);
    }
    return 0;
}