Floyd : 传递闭包

https://www.acwing.com/problem/content/345/

#include <bits/stdc++.h>
using namespace std;
#define IO ios::sync_with_stdio(false);cin.tie(0); cout.tie(0)
inline int lowbit(int x) { return x & (-x); }
#define ll long long
#define pb push_back
#define PII pair<int, int>
#define x first
#define y second
#define inf 0x3f3f3f3f
const int N = 26;
int n, m;
bool g[N][N], d[N][N];
bool st[N];

void floyd() {
    memcpy(d, g, sizeof d);
    for (int k = 0; k < n; ++k) 
        for (int i = 0; i < n; ++i)
            for (int j = 0; j < n; ++j)
                d[i][j] |= d[i][k] && d[k][j];
}

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() {
    IO;
    while (cin >> n >> m, n || m) {
        memset(g, 0, sizeof g);
        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) {
                g[a][b] = 1;
                floyd();
                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;
}