Smallest Number in Infinite Set

You have a set which contains all positive integers `[1, 2, 3, 4, 5, ...]`. Implement the `SmallestInfiniteSet` class: - `SmallestInfiniteSet()` Initializes the **SmallestInfiniteSet** object to contain **all** positive integers. - `int popSmallest()` **Removes** and returns the smallest integer contained in the infinite set. - `void addBack(int num)` **Adds** a positive integer `num` back into the infinite set, if it is **not** already in the infinite set.   **Example 1:** **Input** ["SmallestInfiniteSet", "addBack", "popSmallest", "popSmallest", "popSmallest", "addBack", "popSmallest", "popSmallest", "popSmallest"] [[], [2], [], [], [], [1], [], [], []] **Output** [null, null, 1, 2, 3, null, 1, 4, 5] **Explanation** SmallestInfiniteSet smallestInfiniteSet = new SmallestInfiniteSet(); smallestInfiniteSet.addBack(2); // 2 is already in the set, so no change is made. smallestInfiniteSet.popSmallest(); // return 1, since 1 is the smallest number, and remove it from the set. smallestInfiniteSet.popSmallest(); // return 2, and remove it from the set. smallestInfiniteSet.popSmallest(); // return 3, and remove it from the set. smallestInfiniteSet.addBack(1); // 1 is added back to the set. smallestInfiniteSet.popSmallest(); // return 1, since 1 was added back to the set and // is the smallest number, and remove it from the set. smallestInfiniteSet.popSmallest(); // return 4, and remove it from the set. smallestInfiniteSet.popSmallest(); // return 5, and remove it from the set. ```   **Constraints:** - `1 <= num <= 1000` - At most `1000` calls will be made **in total** to `popSmallest` and `addBack`.