题目链接:http://lightoj.com/volume_showproblem.php?problem=1102
As I am fond of making easier problems, I discovered a problem. Actually, the problem is 'how can you make n by adding k non-negative integers?' I think a small example will make things clear. Suppose n= and k=. There are solutions. They are .
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As I have already told you that I use to make problems easier, so, you don't have to find the actual result. You should report the result modulo 1000,000,007. Input
Input starts with an integer T (≤ ), denoting the number of test cases. Each case contains two integer n ( ≤ n ≤ ) and k ( ≤ k ≤ ). Output
For each case, print the case number and the result modulo . Sample Input
Output for Sample Input Case :
Case :
Case :
Case :
题目大意:求n有顺序的划分为k个数的方案数.
分析:显然这个就是一个组合公式,隔板法。可以把问题转化为x1+x2+…..xk = n 这个多元一次方程上。然后这个解就是C(n+k-1,k-1)
这道题n,k范围都是1e6。
我们可以预处理出阶乘,然后求对应的组合数,注意这里需要取Mod,用下逆元就好啦.
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<math.h>
#include<queue>
#include<stack>
#include <map>
#include <string>
#include <vector>
#include<iostream>
using namespace std;
#define N 3000006
#define INF 0x3f3f3f3f
#define LL long long
#define mod 1000000007
LL arr[N];
void Init()
{
arr[] = ;
for(int i=;i<=N;i++)
{
arr[i] = (arr[i-]*i)%mod;
arr[i] %= mod;
}
}
LL quick(LL a, LL b)
{
a = a%mod;
LL ans = ;
while(b)
{
if(b&)
ans = ans*a%mod;
a = a*a%mod;
b /= ;
}
return ans %mod;
}
LL solve(LL a, LL b, LL c)
{
if(b>a)
return ;
return arr[a] * (quick(arr[b] * arr[a-b],c-)) %mod;///利用乘法逆元
}
int main()
{
Init();
int T;
int con=;
scanf("%d",&T);
while(T--)
{
LL n,k;
scanf("%lld %lld",&n,&k);
printf("Case %d: %lld\n",con++,solve(n+k-,k-,mod));///Cn-k+1(k-1);
}
return ;
}