recursion

Leetcode considers this question to be easy, but it was a bit more complicated than that at first for me. Let’s try solving it three different ways: Whenever I can, I always try to do BFS. It seems much more natural. BFS - 1st solution func isSymmetricBFS(_ root: TreeNode?) -> Bool { var queue: [TreeNode?] = [] queue.append(root) while !queue.isEmpty { let temp = queue queue.removeAll() // 🔑 to make sure you don't expand on `nil` nodes — otherwise it would be endless for node in temp where node !...

Today I’m going to discuss another fun and common challenge. It’s the Longest Common Subsequence a.k.a. LCS. I’ll first focus on discussing a pain point I went through when I was trying to compare the algorithm I deduced on my own vs a few other algorithms I saw online. Our algorithms seemed very similar. Yet different. It made debugging my code based on other code very difficult. This is a very common problem I face when I doing leetcode....

Please read How to Think Recursively before reading this post. This post re-applies the steps mentioned in the previous post on a more challenging question. Question Return all possible ways we can generate a well-formed parenthesis? Examples: if n = 1 then we can only form () if n = 2 then we can form (()) and ()() if n = 3 then we can form ((())), (())(), ()(()), (()()), (), (), () Let’s try applying our 4 steps:...

These articles are about the gotchas I faced when trying to think recursively. The logic in principle should apply to most recursive problems. In this post, I will use the following question as a point of reference: Count how many ways you can climb a staircase. You can jump either one step at a time or two steps at a time. Example if there are 3 stair cases then you can either jump:...