Smallest Number in Infinite Set | MUST DO | OA | TRICKY

https://leetcode.com/problems/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.


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class SmallestInfiniteSet {
public:
    int val=1;
    set<int> PQ;

    SmallestInfiniteSet() {
    }

    int popSmallest() {
        if(PQ.empty() || *PQ.begin()>val){
            int x=val;
            val++;
            return x;
        }
        int x=*PQ.begin();
        PQ.erase(x);
        return x;
    }

    void addBack(int num) {
        if(val>num){
            PQ.insert(num);
        }
    }
};

/**
 * Your SmallestInfiniteSet object will be instantiated and called as such:
 * SmallestInfiniteSet* obj = new SmallestInfiniteSet();
 * int param_1 = obj->popSmallest();
 * obj->addBack(num);
 */