Create an implementation of a Trie structure.
Given an array nums and a number k, return true if there are two distinct indices i and j in the array such that nums[i] == nums[j] and abs(i - j) <= k.
Given a perfect binary tree, count the number of nodes with an algorithm that has less than O(n) time complexity.
Given a binary tree, write an iterator that performs in-order iteration and returns next as well as hasNext. It is guaranteed that next is never called on an empty tree.