invariants

Binary search is simple to explain and notoriously easy to botch (off-by-one, infinite loops). Interviewers look for a correct template plus the variants (boundaries, rotated arrays) that show up constantly. The answer follows.

Gas Station pays off spotting a greedy invariant that turns O(n²) into O(n). Two facts carry the whole solution: total feasibility, and that a failed prefix lets you skip every start within it.