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| #include <bits/stdc++.h>
#pragma optimization_level 3
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")
#define MAX_N 505
#define INF 987654321
#define ll long long
#define pll pair<ll, ll>
#define vpll vector<pll>
#define vall3 vector<array<ll,3>>
#define all5 array<ll,5>
#define vall5 vector<all5>
#define vll vector<ll>
#define vs vector<string>
#define usll unordered_set<ll>
#define vvs vector<vs>
#define vvll vector<vll>
#define all(a) begin(a), end(a)
using namespace std;
string solve(vs &A) {
ll n = A.size();
unordered_map<char, string> eq;
unordered_set<char> us;
unordered_map<char, string> in;
unordered_set<char> out;
for(auto& s : A) {
ll l = 0, r = s.length() - 1;
while(l < s.length() and s[l] == s[0]) l++;
while(r >= 0 and s[r] == s.back()) r--;
if(r < 0) {
eq[s[0]] += s;
} else {
if(s.back() == s[0]) return "IMPOSSIBLE";
char prv = s[l];
while(l <= r) {
if(prv != s[l]) {
if(!us.insert(s[l]).second) return "IMPOSSIBLE";
prv = s[l];
}
l++;
}
if(in.count(s[0]))
return "IMPOSSIBLE";
if(!out.insert(s.back()).second)
return "IMPOSSIBLE";
in[s[0]] = s;
}
}
bool allConn = true;
for(auto& [ch, s] : unordered_map<char,string>(all(eq))) {
if(in.count(ch)) {
in[ch] = s + in[ch];
eq.erase(ch);
}
}
while(allConn) {
allConn = false;
for(auto& [ch, s] : unordered_map<char,string>(all(in))) {
if(!in.count(ch)) continue;
in.erase(ch);
bool conn = true;
while(conn) {
conn = false;
char b = s.back();
if(eq.count(b)) {
s += eq[b];
eq.erase(b);
}
if(in.count(b)) {
s += in[b];
in.erase(b);
conn = true;
allConn = true;
}
}
in[ch] = s;
}
}
string res = "";
us.clear();
for(auto& [_, s] : in)
res += s;
for(auto& [_, s] : eq)
res += s;
char prv = '#';
for(auto& ch : res) {
if(prv == ch) continue;
prv = ch;
if(!us.insert(prv).second)
return "IMPOSSIBLE";
}
return res;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.setf(ios::fixed);
cout.precision(8);
ll tc, n;
string s;
cin >> tc;
for (ll i = 1; i <= tc; i++) {
cin >> n;
vs tower;
while (n--) {
cin >> s;
tower.push_back(s);
}
cout << "Case #" << i << ": " << solve(tower) << '\n';
}
return 0;
}
|
