1Regular Expression MatchingHardSolve → 13Time Needed to Buy TicketsEasySolve → 18Implement Trie (Prefix Tree)MediumSolve → 20Lowest Common Ancestor of a Binary Search TreeMediumSolve → 25Number of Connected Components in an Undirected GraphMediumSolve → 26Flatten Nested List IteratorMediumSolve → 28Kth Smallest Element in a Sorted MatrixMediumSolve → 29Insert Delete GetRandom O(1)MediumSolve → 30Trapping Rain Water IIHardSolve → 31Minimum Genetic MutationMediumSolve → 32Minimum Number of Steps to Make Two Strings AnagramMediumSolve → 33Tweet Counts Per FrequencyMediumSolve → 35Count Student Number in DepartmentsMediumSolve → 38Customer Placing the Largest Number of OrdersEasySolve → 41Number of Black BlocksMediumSolve → 42Masking Personal InformationMediumSolve → 43Design Authentication ManagerMediumSolve → 44Minimum Elements to Add to Form a Given SumMediumSolve → 46Finding the Users Active MinutesMediumSolve → 47Process Tasks Using ServersMediumSolve → 48Remove All Occurrences of a SubstringMediumSolve → 50Random Pick with WeightMediumSolve → 53Minimum Cost to Connect SticksMediumSolve →