uva 10167 - Birthday Cake

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题解：由于解太多,随机抓 A、B, 只要有符合就行了； （首先，Ax+By=0必须表示直线，即A、B不能同时为0；另外，要注意到直线不能过输入中的2N个点；检测点在直线的哪一侧，只需要简单的线性规划的知识）

 #include <cstdio>
 #include <cstdlib>

 int x[], y[];

 int test(int A, int B, int N)
 {
     static int i, pos, neg, tmp;
     pos = , neg = ;
     for (i = *N-; i >= ; i--)
     {
         tmp = A*x[i] + B*y[i];
         if (tmp > ) neg ++;
         else if(tmp < ) pos ++;
         else return ;
     }
     return pos == neg;
 }

 void find(int N)
 {
     int A, B;
     while()
     {
         A = rand()% - ;
         B = rand()% - ;
         if(test(A, B, N))
         {
             printf("%d %d\n", A, B);
             break;
         }
     }
 }

 int main()
 {
     int N, i;
     while(scanf("%d", &N) ==  && N)
     {
         for (i = *N-; i >= ; i--)
             scanf("%d %d", &x[i], &y[i]);
         find(N);
     }
 }

枚举：

 #include<iostream>
 #include<vector>
 using namespace std;
 void bruteforce(int &A, int &B, vector<int> &x, vector<int> &y, int n){
     for (A = -; A <= ; A++){
         for (B = -; B <= ; B++){
             int d = , u = ;
             for (int i = ; i <  * n; i++){
                 if (A*x[i] + B*y[i] > ) u++;
                 if (A*x[i] + B*y[i] < ) d++;
             }
             if (u == n&&d == n) return;
         }
     }
 }
 int main()
 {
     int n;
     while (cin >> n&&n != ){
         vector<int> x(*n, );
         vector<int> y(*n, );
         for (int i = ; i < *n; i++){
             cin >> x[i] >> y[i];
         }
         int A, B;
         bruteforce(A, B, x, y, n);
         cout << A << ' ' << B << endl;
     }
     return ;
 }