341. Flatten Nested List Iterator
考驗著你對於 stack 和 iterator 的應用能力！
315. Count of Smaller Numbers After Self
1D Fenwick Tree 經典應用。
317. Shortest Distance from All Buildings
BFS 應用題！讚！
279. Perfect Squares
Coin change!
218. The Skyline Problem
每個轉折點都會是當下的max height!