Language: C++

#include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define all(c) (c).begin(),(c).end()
#define pb push_back
#define dbg(...) do{cerr<<__LINE__<<": ";dbgprint(#__VA_ARGS__, __VA_ARGS__);}while(0);

using namespace std;

namespace std{template<class S,class T>struct hash<pair<S,T>>{size_t operator()(const pair<S,T>&p)const{return ((size_t)1e9+7)*hash<S>()(p.first)+hash<T>()(p.second);}};template<class T>struct hash<vector<T>>{size_t operator()(const vector<T> &v)const{size_t h=0;for(auto i : v)h=h*((size_t)1e9+7)+hash<T>()(i)+1;return h;}};}
template<class T>ostream& operator<<(ostream &os, const vector<T> &v){os<<"[ ";rep(i,v.size())os<<v[i]<<(i==v.size()-1?" ]":", ");return os;}template<class T>ostream& operator<<(ostream &os,const set<T> &v){os<<"{ "; for(const auto &i:v)os<<i<<", ";return os<<"}";}
template<class T,class U>ostream& operator<<(ostream &os,const map<T,U> &v){os<<"{";for(const auto &i:v)os<<" "<<i.first<<": "<<i.second<<",";return os<<"}";}template<class T,class U>ostream& operator<<(ostream &os,const pair<T,U> &p){return os<<"("<<p.first<<", "<<p.second<<")";}
void dbgprint(const string &fmt){cerr<<endl;}template<class H,class... T>void dbgprint(const string &fmt,const H &h,const T&... r){cerr<<fmt.substr(0,fmt.find(","))<<"= "<<h<<" ";dbgprint(fmt.substr(fmt.find(",")+1),r...);}
typedef long long ll;typedef vector<int> vi;typedef pair<int,int> pi;const int inf = (int)1e9;const double INF = 1e12, EPS = 1e-9;

int timelimit = 0;

int firstDiff(const string &a, const string &b){
	rep(i, min(a.size(), b.size())){
		assert(timelimit++ < 1e8);
		if(a[i] != b[i]) return i;
	}
	return -1;
}

int main(){
	cin.tie(0); cin.sync_with_stdio(0);
	
	int n, len; cin >> n >> len;
	vector<string> v(n); rep(i, n) cin >> v[i];
	vector<vector<bool>> can;
	{
		vector<bool> dp(1, 1);
		can.pb(dp);
		for(int i = n - 1; i >= 0; i--){
			vector<bool> next(dp.size() + v[i].size());
			rep(j, dp.size()) if(dp[j]) next[j] = next[j + v[i].size()] = 1;
			can.pb(next);
			dp = next;
		}
		reverse(all(can));
	}
	vector<bool> from(1, 1);
	string ans;
	
	rep(i, n){
		vector<bool> next(from.size() + v[i].size());
		
		rep(j, from.size()) if(from[j]){
			if(len - j < can[i + 1].size() && can[i + 1][len - j]) next[j] = 1;
			if(len - j - v[i].size() >= can[i + 1].size() || !can[i + 1][len - j - v[i].size()]) continue;
			
			int fd = firstDiff(v[i], ans.substr(j, min(v[i].size(), ans.size() - j));
			if(fd < 0){
				if(j + v[i].size() > ans.size()) ans += v[i].substr(ans.size() - j);
				next[j + v[i].size()] = 1;
				continue;
			}
			
			if(v[i][fd] > ans[j + fd]) continue;
			
			ans.resize(j + v[i].size());
			rep(k, v[i].size()){
				if(k && len - k - j < can[i + 1].size() && can[i + 1][len - k - j]) next[j + k] = 1;
				ans[j + k] = v[i][k];
			}
			next[j + v[i].size()] = 1;
			break;
		}
		from = next;
	}
	cout << ans << endl;
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:52:97: error: expected ')' before ';' token
   52 |                         int fd = firstDiff(v[i], ans.substr(j, min(v[i].size(), ans.size() - j));
      |                                           ~                                                     ^
      |                                                                                                 )