The Core Circuit
A rotation, not a curriculum. Tier One is 9 problems, one per distinct pattern, nothing redundant — together they cover every primitive a practical coding round is likely to touch. The goal is not to have seen them; it is to produce them cold, from a blank file, while talking.
Rep two or three per session so each gets touched roughly weekly. Anything that stumbles gets the full retype ladder — tomorrow, day three, day seven — then leaves the rotation. Once Tier One is cold, Tier Two is the next ring out.
Worth owning, not worth a slot in the spine. Use these as retype-day filler when you want variety — they add pattern coverage the spine mostly already gives you.
The next ring out: patterns Tier One does not touch — intervals, sliding windows, dynamic programming, a monotonic stack, backtracking — plus the grid and tree problems that build on it. Start here when the spine is cold.
Keyword in the prompt → the structure you reach for. Hearing these as triggers is most of the battle.
| “pair / two numbers that sum” | hash map or two pointers |
| “contiguous subarray / substring” | sliding window or prefix sum |
| “sorted array” | binary search or two pointers |
| “shortest / fewest steps / minimum time” | BFS |
| “all paths / does a path exist / flood fill” | DFS |
| “connected / groups / islands” | DFS or union-find |
| “prerequisites / ordering / dependencies” | topological sort |
| “top k / k closest / k largest” | heap (or quickselect) |
| “O(1) get and put” | hash map + linked list |
| “matching / nesting / most recent” | stack |
| “overlapping / conflicting / covered” | sort by start, then sweep |
| “next warmer / next greater” | monotonic stack |
| “longest or shortest window with a property” | sliding window |
| “fewest / min cost / how many ways” | dynamic programming |
| “return all / generate every” | backtracking |