I've been working through coding problems that I can understand when reading solutions but struggle to solve on my own. This series is about breaking down the "why does this actually work" part. Not tutorials. Just honest breakdowns of where my thinking got stuck and what finally unstuck it. The problem that looked familiar LeetCode 316: Remove Duplicate Letters. Given a string like "cbacdcbc" , remove duplicate characters so every letter appears exactly once, and the result is the smallest possible in lexicographical order. The answer here is "acdb" . I'd just finished learning monotonic stacks for Sum of Subarray Minimums. Greedy popping, maintaining order, comparing against the stack top. I recognized the pattern immediately: scan the string, and if the current character is smaller than what's on the stack and the stack-top character appears later in the string, pop it. Classic monotonic stack. So I coded it up, and it was wrong. Where I got stuck My first solution treated uniqueness as a cleanup step.…