Deterministic Finite Automaton (DFA)

 https://en.wikipedia.org/wiki/Deterministic_finite_automaton

Check a valid number or a valid decimal number DFA graph:

class Solution(object):

    def isNumber(self, s):

        # This is the DFA we have designed above

        dfa = [

            {"digit": 1, "sign": 2, "dot": 3},

            {"digit": 1, "dot": 4, "exponent": 5},

            {"digit": 1, "dot": 3},

            {"digit": 4},

            {"digit": 4, "exponent": 5},

            {"sign": 6, "digit": 7},

            {"digit": 7},

            {"digit": 7}

        ]

        

        current_state = 0

        for c in s:

            if c.isdigit():

                group = "digit"

            elif c in ["+", "-"]:

                group = "sign"

            elif c in ["e", "E"]:

                group = "exponent"

            elif c == ".":

                group = "dot"

            else:

                return False

            if group not in dfa[current_state]:

                return False

            

            current_state = dfa[current_state][group]

        

        return current_state in [1, 4, 7]