Hdu2157How many ways??矩阵

给定一个有向图，问从A点恰好走k步（允许重复经过边）到达B点的方案数mod p的值。

详见matrix67博客http://www.matrix67.com/blog/archives/276

#include <cstdio>
#include <algorithm>
#include <iostream>
#include <string.h>
typedef long long LL;
using namespace std;
const LL mod = 1000;
LL n;
struct Matrix
{
    LL m[25][25];
};

Matrix Mul(Matrix a, Matrix b)
{
    Matrix ans;
    for (LL i = 0; i < n; i++){
        for (LL j = 0; j < n; j++){
            ans.m[i][j] = 0;
            for (LL k = 0; k < n; k++){
                ans.m[i][j] += a.m[i][k] * b.m[k][j];
                ans.m[i][j] %= mod;
            }
        }
    }
    return ans;
}

Matrix quick(Matrix a, LL b)
{
    Matrix ans;
    for (LL i = 0; i < n; i++)
    for (LL j = 0; j < n; j++)
        ans.m[i][j] = (i == j);
    while (b){
        if (b & 1) ans = Mul(ans, a);
        a = Mul(a, a);
        b >>= 1;
    }
    return ans;
}

int main()
{
    LL m;
    LL a, b, c, t;
    while (cin >> n >> m,n||m){
        Matrix ans;
        for (LL i = 0; i < n;i++)
        for (LL j = 0; j < n; j++)
            ans.m[i][j] = 0;
        for (LL i = 0; i < m; i++){
            cin >> a >> b;
            ans.m[a][b] = 1;
        }
        cin >> t;
        while (t--){
            cin >> a >> b >> c;
            Matrix sum = quick(ans, c);
            cout << sum.m[a][b] << endl;
        }
    }
    return 0;
}