The Smallest String Concatenation

You're given a list of n strings a₁, a₂, ..., a_n. You'd like to concatenate them together in some order such that the resulting string would be lexicographically smallest.

Given the list of strings, output the lexicographically smallest concatenation.

4
abba
abacaba
bcd
er

abacabaabbabcder

5
x
xx
xxa
xxaa
xxaaa

xxaaaxxaaxxaxxx

3
c
cb
cba

cbacbc

#include<iostream>
#include<string>
#include<algorithm>
#define maxn 50005
using namespace std;
bool cmp(string a,string b)
{return a+b<b+a;
}
int main()
{int n;string str[maxn];cin >>n;for(int i=0;i<n;i++)cin >>str[i];sort(str,str+n,cmp);for(int i=0;i<n;i++)cout <<str[i];return 0; 
}