public int F(string prefix, string suffix)
{
HashSet <int> prefixWords = prefixTree.GetWordIndicesWithPrefix(prefix);
char[] charArray = suffix.ToCharArray();
Array.Reverse(charArray);
suffix = new string(charArray);
HashSet <int> suffixWords = suffixTree.GetWordIndicesWithPrefix(suffix);
// Find the union of words in both and take the largest.
int largest = -1;
foreach (int s in suffixWords)
{
if (prefixWords.Contains(s))
{
largest = Math.Max(s, largest);
}
}
return(largest);
}