I encountered this problem at a past local competition. Here is the brief statement:

Given two string $$$s$$$ of length $$$n$$$ and t of length $$$m$$$ ($$$n$$$, $$$m$$$ $$$\le$$$ $$$10^{5}$$$) find the longest, lexicographically smallest substring of $$$s$$$ that appears in string $$$t$$$ at least $$$k$$$ times. The input is given in a way that there is at least one substring satisfies the constraints.