Mex numbers

Implement a class Mex with the following method:

• add(x): add the number x to the list and return the mex number of the list

The mex number of a list is the smallest non-negative integer that does not occur on the list. For example, the mex number of the list [2,0,3,4,2] is 1.

The method add should take O(1) time on average. You may assume that all the numbers added to the list are non-negative.

In a file mex.py, implement a class Mex according to the following template:

class Mex:
    def __init__(self):
        # TODO

    def add(self, x):
        # TODO

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