HDU 4920 Martix Multiplication (论如何优化稀疏矩阵的运算)

Matrix multiplication

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 131072/131072 K (Java/Others)
Total Submission(s): 5363    Accepted Submission(s): 2050

Problem Description

Given two matrices A and B of size n×n, find the product of them.

bobo hates big integers. So you are only asked to find the result modulo 3.

 

Input

The input consists of several tests. For each tests:

The first line contains n (1≤n≤800). Each of the following n lines contain n integers -- the description of the matrix A. The j-th integer in the i-th line equals A _ij. The next n lines describe the matrix B in similar format (0≤A _ij,B _ij≤10 ⁹).

 

Output

For each tests:

Print n lines. Each of them contain n integers -- the matrix A×B in similar format.

 

Sample Input

1 0 1 2 0 1 2 3 4 5 6 7

 

Sample Output

0 0 1 2 1

 

Author

Xiaoxu Guo (ftiasch)

#include
#include
using namespace std;
#define maxn 805
int n;
/*
本题考验优化能力，
计算矩阵相乘后对结果取模3，
所以所有数字为0，1，2这样，
可以认为其是一个稀疏矩阵，
关键在于把零参与的运算跳过这样，
为此，普通的三次方是行不通的，
所以对运算顺序进行调整，
调整技巧及见代码
*/
long long ma[maxn][maxn],mb[maxn][maxn],res[maxn][maxn];
int main()
{
    while(scanf("%d",&n)==1)
    {
        for(int i=0;i