Intro

A personal problem archive — algorithms, single-cell biology, research design, one problem at a time.

Problems

#	Title	Category	Description	Solution	Added
1	Two Sum	algorithm	Given an array of integers and a target find two numbers that add up to the target	solution	2026-04-14
2	Best Time to Buy and Sell Stock	algorithm	Given daily stock prices find the maximum profit from one buy and one sell	solution	2026-04-16
3	Binary Search	algorithm	Given a sorted array and a target find the target index or return -1	solution	2026-04-17
4	Valid Anagram	algorithm	Given two strings determine whether they contain the same characters with the same counts	solution	2026-04-20
5	Valid Parentheses	algorithm	Given a string of brackets determine whether every bracket is closed in the correct order	solution	2026-04-21
6	Merge Two Sorted Lists	algorithm	Given two sorted linked lists merge them into one sorted linked list	solution	2026-04-22
7	Contains Duplicate	algorithm	Given an array of integers determine whether any value appears at least twice	solution	2026-04-23
8	Valid Palindrome	algorithm	Given a string determine whether it reads the same forward and backward after removing non-alphanumeric characters and ignoring case	solution	2026-04-24
9	Maximum Subarray	algorithm	Given an integer array find the contiguous subarray with the largest sum and return that sum	solution	2026-04-27
10	Move Zeroes	algorithm	Given an integer array move all zeroes to the end while keeping the relative order of non-zero elements	solution	2026-04-28
11	Climbing Stairs	algorithm	Given a staircase with n steps count how many distinct ways you can climb it when each move is 1 or 2 steps	solution	2026-04-29
12	First Bad Version	algorithm	Given n product versions and an API that identifies bad versions find the first bad version	solution	2026-05-05
13	Roman to Integer	algorithm	Given a roman numeral convert it to an integer	solution	2026-05-06
14	Reverse Linked List	algorithm	Given the head of a singly linked list reverse the list and return the new head	solution	2026-05-07
15	Linked List Cycle	algorithm	Given the head of a linked list determine whether the list contains a cycle	solution	2026-05-08
16	Middle of the Linked List	algorithm	Given the head of a singly linked list return the middle node	solution	2026-05-11
17	Plus One	algorithm	Given a non-empty array of digits representing a non-negative integer add one to the integer	solution	2026-05-12
18	Invert Binary Tree	algorithm	Given the root of a binary tree invert the tree and return its root	solution	2026-05-20
19	Maximum Depth of Binary Tree	algorithm	Given the root of a binary tree return its maximum depth	solution	2026-05-21
20	Same Tree	algorithm	Given the roots of two binary trees determine whether they are structurally identical and have the same node values	solution	2026-05-22
21	Symmetric Tree	algorithm	Given the root of a binary tree determine whether it is a mirror of itself	solution	2026-05-26
22	Subtree of Another Tree	algorithm	Given the roots of two binary trees determine whether one tree contains the other as a subtree	solution	2026-05-26
23	Balanced Binary Tree	algorithm	Given the root of a binary tree determine whether it is height-balanced	solution	2026-05-27
24	Diameter of Binary Tree	algorithm	Given the root of a binary tree return the length of the longest path between any two nodes	solution	2026-05-28
25	Minimum Depth of Binary Tree	algorithm	Given the root of a binary tree return its minimum depth	solution	2026-05-29
26	Path Sum	algorithm	Given the root of a binary tree and a target sum determine whether any root-to-leaf path sums to the target	solution	2026-06-01
27	Binary Tree Paths	algorithm	Given the root of a binary tree return all root-to-leaf paths as strings	solution	2026-06-02
28	Sum of Left Leaves	algorithm	Given the root of a binary tree return the sum of all left leaf values	solution	2026-06-03

Not every problem has an obvious answer — and that’s the point. Each one is a small mystery waiting to be solved, one clue at a time. The archive grows slowly, but so does the intuition behind it. No rush. Sherlock didn’t solve cases in a day either.