UVALive 6449 IQ Test --高斯消元？

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;
#define N 4

int f[14];
typedef double Matrix[N][N];
int x,y,z;

void gauss_elimination(Matrix A,int n)
{
    int i,j,k,r;
    for(i=0;i<n;i++)
    {
        //选一行r并与i行交换
        r = i;
        for(j=i+1;j<n;j++)
            if(fabs(A[j][i]) > fabs(A[r][i]))
                r = j;
        if(r != i)
        {
            for(j=0;j<=n;j++)
                swap(A[r][j],A[i][j]);
        }
        //与第i+1~n行进行消元
        for(k=i+1;k<n;k++)
        {
            double f = A[k][i]/A[i][i];  //为了让A[k][i] = 0,第i行乘以的倍数
            for(j=i;j<=n;j++)
                A[k][j] -= f*A[i][j];
        }
    }
    //回代
    for(i=n-1;i>=0;i--)
    {
        for(j=i+1;j<n;j++)
            A[i][n] -= A[j][n]*A[i][j];
        A[i][n] /= A[i][i];
    }
    x = (int)floor(A[0][n]+0.5);
    y = (int)floor(A[1][n]+0.5);
    if(n == 3)
        z = (int)floor(A[2][n]+0.5);
}

int main()
{
    int t,n,i,j;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d",&n);
        memset(f,0,sizeof(f));
        for(i=1;i<=n;i++)
            scanf("%d",&f[i]);
        int ans = Mod;
        int a1,a2,a3;
        int flag;
        if((f[1] == 0 && f[2] == 0) || f[2]%f[1] == 0)
        {
            if(f[1] == 0 && f[2] == 0)
                a1 = 1;
            else
                a1 = f[2]/f[1];
            flag = 1;
            for(i=3;i<=n;i++)
            {
                if(f[i] != a1*f[i-1])
                    flag = 0;
            }
            if(flag)
                ans = a1*f[n];
        }
        if(ans != Mod)
        {
            printf("%d\n",ans);
            continue;
        }
        Matrix A;
        A[0][0] = A[1][1] = f[2];
        A[0][1] = f[1];
        A[1][0] = f[3];
        A[0][2] = f[3];
        A[1][2] = f[4];
        gauss_elimination(A,2);
        flag = 1;
        for(i=3;i<=n;i++)
        {
            if(f[i] != x*f[i-1]+y*f[i-2])
                flag = 0;
        }
        if(flag)
            ans = x*f[n]+y*f[n-1];
        if(ans != Mod)
        {
            printf("%d\n",ans);
            continue;
        }
        A[0][0] = A[1][1] = A[2][2] = f[3];
        A[0][1] = A[1][2] = f[2];
        A[0][2] = f[1];
        A[1][0] = A[2][1] = f[4];
        A[2][0] = f[5];
        A[0][3] = f[4];
        A[1][3] = f[5];
        A[2][3] = f[6];
        gauss_elimination(A,3);
        //printf("%d %d %d\n",x,y,z);
        ans = x*f[n]+y*f[n-1]+z*f[n-2];
        if(ans != Mod)
            printf("%d\n",ans);
    }
    return 0;
}