Killjee and k-th letter

Killjee is trying to unlock a treasure. The key to the treasure is encrypted using a string S and Q queries. In each query, you need to find the K-th letter of a hidden string which is formed from the string S. To form the hidden string, you should sort all substrings of S in lexicographical order and concatenate them. For example, if S = "abc", the hidden string would be "aababcbbcc". (See the s