Topological sort Luogu P1347 sort

Topic link

LG P1347

answer

Title

Find the topological sort.
Pay attention to the three special situations given by the title.

Ideas

The title gives a less than relationship, which can be regarded as an edge.
Take a look at the range given by the title and it is not particularly large, you can come to a side judge once.

1. Output immediately when it hits the ring;
2. Judge whether the number of input nodes is equal to the total number of nodes. If they are not equal, continue to judge, and if they are equal, the output is determined.

Pay attention to the repeated edge, you can skip it directly, otherwise be careful not to record the in-degree repeatedly.
Pay attention to the output format, don’t miss a point QAQ

AC code

#include <bits/stdc++.h>

using namespace std;
//#pragma GCC optimize(2)
#define IO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define ull unsigned long long
#define ll long long
#define rep(i, x, y) for(int i=x;i<=y;i++)
#define mms(x, n) memset(x, n, sizeof(x))
#define mmc(A, tree) memcpy(A, tree, sizeof(tree))
#define eps (1e-8)
#define PI (acos(-1.0))
#define INF (0x3f3f3f3f)
#define mod (ull)(1e9+7)
typedef pair<int, int> P;
int const N = 30;
int e[N][N];
int in[N], tmp[N];
int n, m;
char ch[4];
set<int> st;
bool vis[N];
vector<int> ans;

int topological() {
    
    
    queue<int> q;
    int cnt = 0;
    ans.clear();
    copy(begin(in), end(in), begin(tmp));
    for (int i = 1; i <= n; i++) {
    
    
        if (tmp[i] == 0 && st.find(i) != st.end()) {
    
    
            q.push(i);
            vis[i] = true;
        }
    }
    mms(vis, false);
    bool flag = true;

    while (!q.empty()) {
    
    
        int x = q.front();
        q.pop();

        cnt += 1;
        ans.push_back(x);

        int num = 0;
        for (int y = 1; y <= 26; y++) {
    
    
            if (e[x][y] == 1) {
    
    
                tmp[y] -= 1;
                if (tmp[y] == 0 && !vis[y]) {
    
    
                    num += 1;
                    if (num > 1) flag = false;
                    q.push(y);
                    vis[y] = true;
                }
            }
        }
    }

    if (cnt < st.size()) return -1;
    else if (cnt == n && flag) return 1;
    return 0; // 没有必要，至少不写有点不舒服QAQ
}

int main() {
    
    
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
#endif
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++) {
    
    
        scanf("%s", ch);
        int x = ch[0] - 'A' + 1;
        int y = ch[2] - 'A' + 1;
        st.insert(x);
        st.insert(y);
        if (e[x][y] == 1) continue;
        e[x][y] = 1;
        in[y] += 1;
        int ret = topological();
        if (ret == -1) {
    
    
            printf("Inconsistency found after %d relations.", i);
            return 0;
        } else if (ret == 1) {
    
    
            printf("Sorted sequence determined after %d relations: ", i);
            for (auto item : ans) printf("%c", 'A' + item - 1);
            printf(".");
            return 0;
        }
    }
    printf("Sorted sequence cannot be determined.");
    return 0;
}