LeetCode Medium Problems 75¶

(a.k.a. Welcome to the Algorithm Hunger Games)

Now that you’ve survived Easy mode, it’s time to enter Medium — the level where your code works on sample tests only.

Medium problems are like business meetings:

Everyone looks calm,
But deep down, nobody knows what’s happening.

⚔️ What “Medium” Really Means¶

You’ll meet algorithms with names like “Dynamic Programming,” “Sliding Window,” and “Backtracking.” Each one sounds like a yoga pose but feels like a mild existential crisis.

🧩 Classic Medium Problems¶

Problem	Concept	Comedy Summary
3Sum	Two pointers	“The sequel to Two Sum — now featuring drama.”
Add Two Numbers (Linked Lists)	Linked list arithmetic	“Because normal addition was too mainstream.”
Group Anagrams	Hash maps	“Because spelling is hard — even for computers.”
Longest Substring Without Repeating Characters	Sliding window	“Teaches you commitment issues.”
Binary Tree Level Order Traversal	BFS	“A data structure family reunion.”
Kth Smallest Element in a BST	In-order traversal	“Counting leaves like it’s autumn.”
Word Search	Backtracking	“Hide-and-seek for letters — exhausting but fun.”
Combination Sum	Recursion	“The mathematical buffet problem: take everything that fits.”
Top K Frequent Elements	Heap	“Who are the influencers of your array?”

🧠 Medium Survival Tips¶

Draw things. Half the time, your brain just wants a picture.
Name your variables properly. x, xx, and xxx aren’t debugging-friendly.
Expect failure. If your first submission passes, check if you’re dreaming.
Read solutions, not to copy — but to steal wisdom.

Pro Tip: Medium problems are like dating — the key is consistency, not speed.

“Medium isn’t about difficulty. It’s about humility.” 😇

# Your code here

Exercises¶

Exercise¶

Implement two_sum(nums, target) that returns a tuple of indices (i, j) whose values sum to target, or None if no pair exists.