02-18 [LeetCode][287. Find the Duplicate Number] 9 Approaches：Brute Force, Count, Hash, In-place Marked, Sorting, Index Sort, Binary Search, Bit Manipulation, Fast Slow Pointers
02-16 [LeetCode][114. Flatten Binary Tree to Linked List] 3 Approaches: PreOrder, Iteration and Recursion
02-16 [LeetCode][1780. Check if Number is a Sum of Powers of Three] 3 Solutions: Tricky, Recursion and Base 3 Convertion