One puzzle a day. No code required.
Burning ropes, bulb switches, horse races. These are the questions interviewers reach for when they want to watch you think, not recite. Each puzzle ships with graduated hints and a solution argued from first principles.
Today's puzzle
Two ropes, each burns in an hour. Measure 45 minutes.
The classic burning-rope interview puzzle: two ropes that each take an hour to burn, but burn unevenly, and a single lighter. How do you measure exactly 45 minutes? The trick is realising what stays fixed when the burn rate does not.
Take on the puzzleWhy puzzles, on an algorithms site?
Because the skill they test is the same one the curriculum trains: taking a fuzzy problem, finding the constraint that actually matters, and reasoning to a provable answer. A rope that burns unevenly is a warm-up for an invariant argument; the egg-drop question is search-space thinking wearing a costume. Interviewers at large companies have mostly stopped scoring these questions directly, but they still ask cousins of them, and the habit of mind they reward, working from what is fixed toward what is unknown, is the habit every hard interview rewards.
A new puzzle is posted each day. Read the question, close the tab, and think while you make coffee. The hints are ordered so the first one nudges and the last one nearly spoils; open them one at a time and stop the moment something clicks.
The archive
6 puzzles postedSolved them all? The problem walkthroughs carry the same habit of mind into real code.