You're given a binary array nums and an integer k — you may flip up to k zeros to ones. Return the length of the longest contiguous run of 1s you can achieve. Re-checking every possible starting point from scratch is wasteful: as the window slides forward, most of its elements stay the same. A variable-size sliding windowVariable-Size Sliding WindowA window whose right edge always advances, but whose left edge only advances when a constraint is violated — so the window grows and shrinks in response to what it currently holds, rather than staying a fixed size. tracks how many 0s the current window is "spending" as flips; whenever that count exceeds k, the left edge shrinks just enough to bring it back within budget, and every position is visited by each pointer at most once.