Variable-Size Window

Variable-size windows expand when adding elements is beneficial and shrink when a constraint is violated.

Template for "find longest subarray with condition":

left = 0
for right from 0 to n-1:
    // Expand: add arr[right] to window state

    while window violates condition:
        // Shrink: remove arr[left] from window state
        left = left + 1

    // Window [left, right] is valid
    update answer with (right - left + 1)

core idea: right always moves forward. left only moves forward (never backward). Each element is added once and removed at most once. Total: $O(n)$.