感觉就是模拟它的两个过程就行了啊

class SmallestInfiniteSet:

    def __init__(self):
        self.small = 1
        self.delset = set()

    def popSmallest(self) -> int:
        ans = self.small
        self.delset.add(self.small)
        while self.small in self.delset:
            self.small += 1
        return ans

    def addBack(self, num: int) -> None:
        if num not in self.delset:
            return
        self.delset.remove(num)
        if num < self.small:
            self.small = num