代码

#define idx(i, j) ((i) * n + (j))
#include <algorithm>
void matrix_multiply(int n, const double *A, const double *B, double *C) {
for (int i=0; i<n; ++i) for (int j=i; ++j<n; ) std::swap(((double*)B)[idx(i,j)],((double*)B)[idx(j,i)]);
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			double t = 0;
			for (int k = 0; k < n; k++) {
				t += A[idx(i, k)] * B[idx(j, k)];
			}
			C[idx(i, j)] = t;
		}
	}
}

评测结果