uva 10167

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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[100], y[100];

int test(int A, int B, int N)
{
    static int i, pos, neg, tmp;
    pos = 0, neg = 0;
    for (i = 2*N-1; i >= 0; i--)
    {
        tmp = A*x[i] + B*y[i];
        if (tmp > 0) neg ++;
        else if(tmp < 0) pos ++;
        else return 0;
    }
    return pos == neg;
}

void find(int N)
{
    int A, B;
    while(1)
    {
        A = rand()%1001 - 500;
        B = rand()%1001 - 500;
        if(test(A, B, N))
        {
            printf("%d %d
", A, B);
            break;
        }
    }
}

int main()
{
    int N, i;
    while(scanf("%d", &N) == 1 && N)
    {
        for (i = 2*N-1; i >= 0; 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 = -500; A <= 500; A++){
        for (B = -500; B <= 500; B++){
            int d = 0, u = 0;
            for (int i = 0; i < 2 * n; i++){
                if (A*x[i] + B*y[i] > 0) u++;
                if (A*x[i] + B*y[i] < 0) d++;
            }
            if (u == n&&d == n) return;
        }
    }
}
int main()
{
    int n;
    while (cin >> n&&n != 0){
        vector<int> x(2*n, 0);
        vector<int> y(2*n, 0);
        for (int i = 0; i < 2*n; i++){
            cin >> x[i] >> y[i];
        }
        int A, B;
        bruteforce(A, B, x, y, n);
        cout << A << ' ' << B << endl;
    }
    return 0;
}