Lexicographically Smallest Generated String

You are given two strings, str1 and str2, of lengths n and m, respectively.

A string word of length n + m - 1 is defined to be generated by str1 and str2 if it satisfies the following conditions for each index 0 <= i <= n - 1:

• If str1[i] == 'T', the substring of word with size m starting at index i is equal to str2, i.e., word[i..(i + m - 1)] == str2.
• If str1[i] == 'F', the substring of word with size m starting at index i is not equal to str2, i.e., word[i..(i + m - 1)] != str2.

Return the lexicographically smallest possible string that can be generated by str1 and str2. If no string can be generated, return an empty string "".