Leetcode 995. Minimum Number of K Consecutive Bit Flips

Given a binary array and an integer k, compute the minimum number of length-k subarray flips needed to make all elements 1 (or return -1 if impossible). The core challenge is handling overlapping flips efficiently — use a greedy left-to-right approach that tracks the current parity of applied flips (via a difference array or sliding window) to decide when to flip.