2336\. Smallest Number in Infinite Set

Medium

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`.