bzoj3505 [Cqoi2014]数三角形

题目链接

先算在n*m个点中任选3个的方案数，再减去三点共线的方案数

我为什么要做这种水题？因为我很弱

 1 #include<algorithm>
 2 #include<iostream>
 3 #include<cstdlib>
 4 #include<cstring>
 5 #include<cstdio>
 6 #include<string>
 7 #include<cmath>
 8 #include<ctime>
 9 #include<queue>
10 #include<stack>
11 #include<map>
12 #include<set>
13 #define rre(i,r,l) for(int i=(r);i>=(l);i--)
14 #define re(i,l,r) for(int i=(l);i<=(r);i++)
15 #define Clear(a,b) memset(a,b,sizeof(a))
16 #define inout(x) printf("%d",(x))
17 #define douin(x) scanf("%lf",&x)
18 #define strin(x) scanf("%s",(x))
19 #define LLin(x) scanf("%lld",&x)
20 #define op operator
21 #define CSC main
22 typedef unsigned long long ULL;
23 typedef const int cint;
24 typedef long long LL;
25 using namespace std;
26 void inin(int &ret)
27 {
28     ret=0;int f=0;char ch=getchar();
29     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=1;ch=getchar();}
30     while(ch>=‘0‘&&ch<=‘9‘)ret*=10,ret+=ch-‘0‘,ch=getchar();
31     ret=f?-ret:ret;
32 }
33 int n,m;
34 int C(int a,int b=3)
35 {
36     return a*(a-1)*(a-2)/6;
37 }
38 int gcd(int a,int b){int c;while(a%b)c=a%b,a=b,b=c;return b;}
39 int main()
40 {
41     inin(n),inin(m);
42     n++,m++;
43     LL now=n*m;
44     LL ans=1LL*now*(now-1)*(now-2)/6;
45     ans-=1LL*m*C(n,3)+1LL*n*C(m,3);
46     re(i,1,n-1)
47         re(j,1,m-1)
48         {
49             int temp=gcd(i,j);
50             if(temp>=2)
51             ans-=1LL*(temp-1)*2*(n-i)*(m-j);
52         }
53     cout<<ans;
54     return 0;
55 }