KOI: Cecil

#include <bits/stdc++.h>
using namespace std;

const int N = 100 + 5;

set<int> adj[26];
string str[N];
int inDeg[26];

int main(){

    int n;
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i){
        cin >> str[i];
    }
    for(int i = 1; i <= n - 1; ++i){
        for(int j = i + 1; j <= n; ++j){
            int len = min(str[i].size(), str[j].size());
            for(int k = 0; k < len; ++k){
                if(str[i][k] != str[j][k]){
                    int u = str[i][k] - 'a';
                    int v = str[j][k] - 'a';
                    if(adj[u].insert(v).second){
                        ++inDeg[v];
                    }
                    break;
                }
            }
        }
    }
    priority_queue<int, vector<int>, greater<int>> pq;
    for(int i = 0; i < 26; ++i){
        if(inDeg[i] == 0){
            pq.push(i);
        }
    }
    string ans;
    while(!pq.empty()){
        int u = pq.top();
        pq.pop();
        ans.push_back((char)(u + 'a'));
        for(int v : adj[u]){
            --inDeg[v];
            if(inDeg[v] == 0){
                pq.push(v);
            }
        }
    }
    if(ans.size() < 26){
        cout << "-1";
    } else {
        cout << ans;
    }

    return 0;
}

// Rework: O(N^2) Solution
#include <bits/stdc++.h>
using namespace std;

const int N = 100 + 5;

vector<int> adj[26];
string str[N];
int inDeg[26];

int main(){

    int n;
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i){
        cin >> str[i];
    }
    for(int i = 1; i < n; ++i){
        int j = i + 1;
        int len = min(str[i].size(), str[j].size());
        for(int k = 0; k < len; ++k){
            if(str[i][k] != str[j][k]){
                char u = str[i][k] - 'a';
                char v = str[j][k] - 'a';
                adj[u].push_back(v);
                ++inDeg[v];
                break;
            }
        }
    }
    priority_queue<int, vector<int>, greater<int>> pq;
    string ans;
    for(int u = 0; u < 26; ++u){
        if(inDeg[u] == 0){
            pq.push(u);
        }
    }
    while(!pq.empty()){
        int u = pq.top();
        pq.pop();
        ans.push_back((char)(u + 'a'));
        for(int v : adj[u]){
            if(--inDeg[v] == 0){
                pq.push(v);
            }
        }
    }
    if(ans.size() < 26){
        cout << "-1";
    } else {
        cout << ans;
    }

    return 0;
}