Leetcode 2014. Longest Subsequence Repeated k Times

Find the longest string seq such that seq repeated k times (seq*k) is a subsequence of s, returning the lexicographically largest one on ties (or empty if none). The core challenge is searching the combinatorial space of subsequences under subsequence-check constraints (lowercase alphabet, n up to ~2000, k up to 2000) with pruning/position-based preprocessing.