invariants

Binary search is easy to describe and famously easy to get wrong (off-by-one, infinite loops). The signal is a correct template plus the variants (boundaries, rotated arrays) that come up constantly. Here is the answer.

Gas Station rewards 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 inside it.