Coin Streak Probability Interview Questions
Coin streak probability prep for quant interviews, including fixed-length streaks, pattern waiting times, overlap, recursion, and common mistakes.
Candidates practicing state recursion and pattern overlap in coin-flip prompts.
Fixed streaks versus waiting
A fixed-length streak question asks whether a pattern occurs in a set number of flips. A waiting-time question asks how long until the pattern appears. These need different methods.
Overlap matters
Patterns can overlap with themselves. Waiting for HH differs from waiting for HT because after seeing H, the next flip may preserve or reset progress differently. State diagrams keep this straight.
Concrete example
For waiting until HH, after one H you are close, and another H finishes. A tail resets. For HT, after H you are close, but another H still leaves you with a useful H state. The transition structure drives the expectation.
Build states
Define states by how much of the target pattern has been matched. Then write equations for expected time or hitting probability. Boundary state means the pattern is complete.
Practice path
Start with runs of two heads, then patterns like HT, HHT, and HTH. After each, ask what partial progress remains after a failed next flip.
Common mistakes
Candidates often assume all same-length patterns have equal waiting time. They do not when overlap differs. The state definition is the protection against that mistake.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.