It is sufficient to sort the sequence of strings in ascending order of their lengths.

For example, one can specify the comparing as the function that one that compares the lengths.

Sample code

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

int main() {
	int n;
	cin >> n;
	vector<string> s(n);
	for (int i = 0; i < n; i++) cin >> s[i];
	sort(s.begin(), s.end(), [](string x, string y) { return x.size() < y.size(); });
	string ans = "";
	for (int i = 0; i < n; i++) ans += s[i];
	cout << ans << '\n';
}