Leetcode 2336. Smallest Number in Infinite Set

Maintain an infinite set of positive integers that supports popSmallest(), which removes and returns the current smallest element, and addBack(num), which re-inserts a number if it's missing. The challenge is to efficiently track the next unused integer while handling reinserts so popSmallest always returns the smallest available (constraints: num ≤ 1000, ≤1000 operations).