題意：給出ｎ個字串，問有沒有一種字元的替換方案使得所有的字串按照字典序遞增排列。

只要使得任意連續的兩個有序即可，如果字元ａ必須比字元ｂ字典序小就建一條ａ到ｂ的有向邊，然後通過一邊拓撲排序輸出結果即可。

#include <bits/stdc++.h>
using namespace std;
#define maxn 33

int in[maxn];
int g[maxn][maxn];
int n;
string a[105];

void add_edge (char a, char b) { 
    if (!g[a-'a'][b-'a']) in[b-'a']++;
    g[a-'a'
 
][b-'a'] = 1;
    return ;
}

bool build (int i, int j) {
    int l1 = a[i].length (), l2 = a[j].length ();
    for (int x = 0; x < min (l1, l2); x++) {
        if (a[i][x] != a[j][x]) {
            add_edge (a[i][x], a[j][x]);
            return 1;
        }
    }
    if (l2 <= l1) return 0;
    return
 
 1;
}

queue <int> q;
vector <int> ans;
bool topo () {
    ans.clear ();
    while (!q.empty ()) q.pop ();
    int num = 0;
    for (int i = 0; i < 26; i++) if (!in[i]) {
        q.push (i);
        num++;
    }
    while (!q.empty ()) {
        int u = q.front (); q.pop ();
        ans.push_back (u);
        for
 
 (int i = 0; i < 26; i++) if (g[u][i] && in[i]) {
            in[i]--;
            if (!in[i]) {
                q.push (i);
                num++;
            }
        }
    }
    if (num != 26) return 0;
    for (int i = 0; i < 26; i++) cout << (char)(ans[i]+'a'); cout << endl;
    return 1;
}

int main () {
    memset (g, 0, sizeof g);
    memset (in, 0, sizeof in);
    cin >> n;
    for (int i = 0; i < n; i++) cin >> a[i];
    for (int i = 0; i < n-1; i++) {
        int j = i+1;
        bool ok = build (i, j);
        if (!ok) {
            cout << "Impossible" << endl;
            return 0;
        }
    }
    if (!topo ()) cout << "Impossible" << endl;
    return 0;
}