n阶矩阵乘以n阶矩阵的朴素做法

n阶矩阵 * n阶矩阵
k: 1 to n 
c[i][j] = sum (a[i][k] * c[k][j])

for (int i = 0; i < n; ++i)
    for (int j = 0; j <n; ++j){
        c[i][j] = 0;
        for (int k = 0; k < n; ++k)
            c[i][j] += c[i][k] * c[k][j];
    }