Variable-Size Windows: The Left Pointer Wakes Up

Concepts: pyVariableWindow, pyTwoPointer

In a fixed-width window, the left pointer is a zombie. It moves exactly one step for every step the right pointer takes. In a variable-size window, the left pointer has agency. It moves zero or more steps after each expansion of the right pointer, driven entirely by whether the window constraint is satisfied. This is the conceptual leap. The right pointer always advances. The left pointer advances only when the window is invalid. The gap between them, right - left + 1, is the window size, and it varies dynamically. The Canonical Variable Window: Longest Substring Without Repeating Characters This is the single most common intermediate sliding window problem. Every FAANG loop I have seen at the senior level either asks this directly or uses a variant of it. The constraint is 'no character a