floyd 算法   如果存在无数条路  则存在a->a的路  a->b的路径数等于 a->i 和 i->b(0=<i<=_max) 路径数的乘积和

#include<cstdio>

#include<cstring>

#include<algorithm>

using namespace std;

const int maxn = 30+5;

int n, g[maxn][maxn],ca;

int main()

{

    ca = 0;

    while (scanf("%d", &n) == 1)

    {

        int _max = 0;

        memset(g, 0, sizeof(g));

        for (int i = 0; i < n; i++)

        {

            int x, y;

            scanf("%d%d", &x, &y);

            g[x][y] = 1;

            _max = max(_max, max(x, y));

        }

        for (int k = 0; k <= _max; k++)

            for (int i = 0; i <= _max; i++)

                for (int j = 0; j <= _max; j++)

                    g[i][j] += g[i][k]*g[k][j];

        for (int k = 0; k <= _max; k++)

            if (g[k][k])

                for (int i = 0; i <= _max; i++)

                    for (int j = 0; j <= _max; j++)

                        if (g[i][k] && g[k][j])

                            g[i][j] = -1;

        printf("matrix for city %d\n", ca++);

        for (int i = 0; i <= _max; i++)

        {

            for (int j = 0; j <= _max; j++)

            {

                if (j) putchar(' ');

                printf("%d", g[i][j]);

            }

            puts("");

        }

    }

    return 0;

}