Binary Search Interview Questions & Solutions

Everyone learns binary search on a sorted array, but interviews reward the candidates who see it everywhere else. The real pattern is more general: whenever you can ask a yes/no question whose answer is monotonic — true, true, true, then false, false — you can binary-search for the boundary in O(log n). That reframing is what lets you "binary search on the answer" for problems that never mention a sorted array at all.

The solutions here start with the fundamentals done right — because off-by-one and infinite-loop bugs in the basic template are shockingly common under pressure — then build up to the interview favorites: searching in rotated sorted arrays, finding boundaries and peak elements, and the "minimum capacity / smallest feasible value" problems solved by binary-searching the answer space. We're deliberate about loop invariants and the `left`/`right`/`mid` bookkeeping so your implementation is correct the first time. Get comfortable here and a surprising number of "hard" optimization problems reduce to a clean logarithmic search.