bzoj3505 组合数

http://www.lydsy.com/JudgeOnline/problem.php?id=3505

这道题斜边一开始想成o(n^2m^2)的了……实际上是一道很水的题.注意1000*1000的范围，组合数最好递推.

/**************************************************************    Problem: 3505    User: MaxMercer    Language: C++    Result: Accepted    Time:516 ms    Memory:32228 kb****************************************************************/#include<stdio.h>#include<algorithm>typedef long long dnt;dnt ans,c[1005005][4],tmp;int gcd(int a,int b){return b==0?a:gcd(b,a%b);}int n,m;void cc(){    c[0][0]=1;    for(register int i=1;i<=n*m;i++){        c[i][0]=1;        for(int j=1;j<=3;j++) c[i][j]=c[i-1][j-1]+c[i-1][j];    }}int main(){    scanf("%d%d",&n,&m);    n++,m++;    cc();    ans=c[n*m][3];      ans-=c[m][3]*n;    ans-=c[n][3]*m;    for(int i=1;i<n;i++)     for(int j=1;j<m;j++){         tmp=gcd(i,j)+1;         if(tmp>2) ans-=(tmp-2)*2*(n-i)*(m-j);      }    printf("%lld",ans);}