Determine if a number is a 'happy number'. A happy number eventually reaches 1 when replaced by the sum of the squares of its digits, repeating the process until it does.

Given a number n and an integer k, find the kth lexicographical permutation of the sequence 1, 2, ..., n. Essentially, you need to generate the kth permutation without explicitly calculating all the preceding permutations.

Determine if one binary tree is a subtree of another binary tree. That is, check if the structure and node values of the smaller tree exactly match a portion of the larger tree.