Stopping Time Quant Interview Questions
How to solve quant interview questions where a process stops after a condition, threshold, pattern, or decision.
Candidates practicing coin, dice, card, and game stopping rules.
Name the stopping condition
A stopping-time problem is defined by when the process ends. State that condition before solving. Does the game stop after a pattern, a threshold, a success, a failure, or a choice by the player?
Choose the state
The state should summarize progress toward stopping. For a streak problem, state the current streak length. For a random walk, state the position. For a reroll game, state the observed value and remaining choices.
Expected time versus payoff
Some stopping questions ask how long the process takes. Others ask what the game is worth. Expected time and expected payoff can use similar states, but the equations and boundary values differ.
Concrete example
If a coin is flipped until the first head, the expected number of flips E satisfies E = 1 + 0.5E, because after a tail you are back where you started. Solving gives E = 2.
Practice variants
After solving one stopping problem, change the pattern, threshold, or payoff. This forces you to rebuild the state instead of memorizing the answer.
Common mistakes
Candidates often forget that a failed attempt may leave partial progress. Pattern waiting-time problems are especially sensitive to this. Draw states when overlap matters.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.