Studious Student Problem Analysis

(http://leetcode.com/2011/01/studious-student-problem-analysis.html)

You've been given a list of words to study and memorize. Being a diligent student of language and the arts, you've decided to not study them at all and instead make up pointless games based on them. One game you've come up with is to see how you can concatenate the words to generate the lexicographically lowest possible string.

Input:

As input for playing this game you will receive a text file containing an integer N, the number of word sets you need to play your game against. This will be followed by N word sets, each starting with an integer M, the number of words in the set, followed by M words. All tokens in the input will be separated by some whitespace and, aside from N and M, will consist entirely of lowercase letters.

Output:

Your submission should contain the lexicographically shortest strings for each corresponding word set, one per line and in order.

Constraints:

1 <= N <= 100

1 <= M <= 9

1 <= all word lengths <= 10

--

It's not right to sort and concatenate each individual word together to form the lexicographically smallest string. For example, inputs are "zza zz".

If no word appears to be a prefix of any other words, then the simple sort + concatenate must yield the smallest dictionary order string.

We re-define the order relation of two words, s1 and s2, as:

s1 is less than s2 iff
s1 + s2 < s2 + s1

Code:

bool compareSort(const string& s1, const string& s2)
{
    return s1 + s2 < s2 + s1;
}

int main()
{
    string words[];
    int N, M;

    cin >> N;
    for (int i = ; i < N; i++)
    {
        cin >> M;

        for (int j = ; j < M; i++)
            cin >> words[j];

        sort(words, words+M, compareSort);

        for (int j = ; j < M; j++)
            cout << words[j];
        cout << endl;
    }
}