求逆矩阵

void inv(mat &x){

    int n = 2, is[2], js[2];
    memset(is, 0, sizeof(is));
    memset(js, 0, sizeof(js));
    for (int k = 0; k < n; k++)
    {
        for (int i = k, j; i < n; i++)
        {
            for (j = k; j < n && !x.a[i][j]; j++);
            is[k] = i, js[k] = j;
        }
        for (int i = 0; i < n; i++)
            swap(x.a[k][i],x.a[is[k]][i]);
        for (int i = 0; i < n; i++)
            swap(x.a[i][k], x.a[i][js[k]]);
        if (x.a[k][k]==0){
            puts("No Solution"); exit(0) ;
        }
        x.a[k][k] = inv(x.a[k][k]);
        for (int j = 0; j < n; j++)
            if (j != k)
                (x.a[k][j] *= x.a[k][k]) %= mod; 
        for (int i = 0; i < n; i++) 
            if (i != k)
            {
                int temp = x.a[i][k];
                x.a[i][k] = 0;
                for(int j = 0; j < n; ++j)
                    (x.a[i][j] += mod - x.a[k][j] * temp % mod) %= mod;
            }
    }
    for (int k=n-1; k>=0; k--){
        for (int i = 0; i < n; i++)
            swap(x.a[js[k]][i], x.a[k][i]);
        for (int i = 0; i < n; i++)
            swap(x.a[i][is[k]],x.a[i][k]);
    }
}