ACwing343.排序

1.Floyd写法：

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 26;

int n, m;
bool d[N][N];
bool st[N];

int check()
{
    for (int i = 0; i < n; i ++ )
        if (d[i][i])
            return 2;

    for (int i = 0; i < n; i ++ )
        for (int j = 0; j < i; j ++ )
            if (!d[i][j] && !d[j][i])
                return 0;

    return 1;
}

char get_min()
{
    for (int i = 0; i < n; i ++ )
        if (!st[i])
        {
            bool flag = true;
            for (int j = 0; j < n; j ++ )
                if (!st[j] && d[j][i])
                {
                    flag = false;
                    break;
                }
            if (flag)
            {
                st[i] = true;
                return 'A' + i;
            }
        }
}

int main()
{
    while (cin >> n >> m, n || m)
    {
        memset(d, 0, sizeof d);

        int type = 0, t;
        for (int i = 1; i <= m; i ++ )
        {
            char str[5];
            cin >> str;
            int a = str[0] - 'A', b = str[2] - 'A';

            if (!type)
            {
                d[a][b] = 1;
                for (int x = 0; x < n; x ++ )
                {
                    if (d[x][a]) d[x][b] = 1;
                    if (d[b][x]) d[a][x] = 1;
                    for (int y = 0; y < n; y ++ )
                        if (d[x][a] && d[b][y])
                            d[x][y] = 1;
                }
                type = check();
                if (type) t = i;
            }
        }

        if (!type) puts("Sorted sequence cannot be determined.");
        else if (type == 2) printf("Inconsistency found after %d relations.\n", t);
        else
        {
            memset(st, 0, sizeof st);
            printf("Sorted sequence determined after %d relations: ", t);
            for (int i = 0; i < n; i ++ ) printf("%c", get_min());
            printf(".\n");
        }
    }

    return 0;
}

因为更新d数组的时候，两重循环都是从0到n-1，所以也可以这样写：

for (int x = 0; x < n; x ++ )
                {
                    if (d[x][a]) d[x][b] = 1;
                    for (int y = 0; y < n; y ++ )
                    {
                        if(d[b][y]) d[a][y] = 1;
                        if (d[x][a] && d[b][y])
                            d[x][y] = 1;
                    }
                        
                }