class Mex:
    def __init__(self):
        self.set = {0}
        self.lowest_missing = 0

    def add(self, x):
        self.set.add(x)

        if x != self.lowest_missing:
            return self.lowest_missing

        for y in range(self.lowest_missing, len(self.set) + 1):
            if y in self.set:
                continue
            else:
                self.lowest_missing = y
                return self.lowest_missing

        return self.lowest_missing


if __name__ == "__main__":
    m = Mex()
    print(m.add(1))  # 0
    print(m.add(3))  # 0
    print(m.add(4))  # 0
    print(m.add(0))  # 2
    print(m.add(5))  # 2
    print(m.add(2))  # 6
    # m = Mex()
    # for i in range(10**5):
    #     print(m.add(i))