Given an array nums, return the length of the longest contiguous subarray that contains at most 2 distinct values. Checking every starting index and re-scanning forward works, but it forgets everything the previous start already learned about the array. The sliding windowSliding WindowMaintaining a running result over a contiguous range that grows or shrinks one element at a time, instead of recomputing the result for every range from scratch. technique keeps a frequency map of the values currently in view: grow the window by moving the right edge forward, and whenever a 3rd distinct value sneaks in, shrink from the left — dropping counts back to zero — until the window is valid again. Every element enters and leaves the window at most once, so the whole scan runs in O(n).