[CF #290-C] Fox And Names (拓扑排序)

 1 #include <cstdio>
 2 #include <algorithm>
 3 #include <bitset>
 4 #include <set>
 5 #include <vector>
 6 #include <iterator>
 7 #include <cstring>
 8 #include <map>
 9 #include <cctype>
10 #include <iostream>
11 #include <string>
12 #include <stdexcept>
13 using namespace std;
14 
15 int n;
16 string names[111];
17 bool hasAns = true;
18 vector<int> G[33];
19 int c[33],topo[33],t;
20 
21 void solve(int l,int r,int x){
22     if( l>=r ) return;
23     if( !hasAns ) return;
24     for(int i=l;i<=r;i++){
25         if( x>=names[i].size() ) {
26             hasAns = false;
27             return;
28         }
29         int j = i+1;
30         for(;j<=r;j++){
31             if( names[i][x]==names[j][x] ){
32                 continue;
33             } else {
34                 break;
35             }
36         }
37         while( names[i].size()-1<x+1&&i<j-1 ) i++;
38         solve(i,j-1,x+1);
39         i = j-1;
40     }
41 
42     for(int i=l;i<r;i++) {
43         if( names[i][x]==names[i+1][x] ) continue;
44         G[names[i][x]-‘a‘].push_back(names[i+1][x]-‘a‘);
45     }
46 }
47 
48 bool dfs(int u){
49     c[u] = -1;
50     for(int i=0;i<G[u].size();i++){
51         int v = G[u][i];
52         if( v<0 ) continue;
53         if( c[v]<0 ) return false;
54         else if(!c[v] && !dfs(v) ) return false;
55     }
56     c[u] = 1; topo[--t] = u;
57     return true;
58 }
59 
60 bool toposort(){
61     t = 26;
62     memset(c,0,sizeof(c));
63     for(int u=25;u>=0;u--) if(!c[u])
64         if(!dfs(u)) return false;
65     return true;
66 }
67 
68 int main(){
69     cin >> n;
70     for(int i=0;i<n;i++) cin >> names[i];
71     solve(0,n-1,0);
72     if( !hasAns ) {
73         puts("Impossible");
74         return 0;
75     }
76     bool hasAns = toposort();
77     if( !hasAns ) {
78         puts("Impossible");
79         return 0;
80     }
81     for(int i=0;i<26;i++){
82         putchar(‘a‘+topo[i]);
83     }
84     puts("");
85 
86     return 0;
87 }