Solves valid number parsing for: https://oj.leetcode.com/problems/valid-number/. Parsing strings for valid integer value is something you use in your Programming Languages. Here -- I tried to model the code similar to how I would conceptually think of the problem. In this case -- Python was great for providing a minimal interface

solve.py

# use a state machine model
class State:
    def __init__(self, name, chars):
        self.name = name
        self.chars = chars

    def __contains__(self, item):
        return item in self.chars

    def __repr__(self):
        return "State('%s', '%s')" % (self.name, self.chars)

# These represent all the different states of the we are in and the values that transition into that state.
START = State('start', ' ')
WHOLE = State('whole', '0123456789')
DOT = State('dot', '.')
FRACTIONAL = State('frac', '0123456789')
E = State('e', 'e')
E_SIGN = State('esign', '+-')
E_NUMBER = State('e_number', '0123456789')
SIGN = State('neg', '-+')
END = State('end', ' ')

DEBUG = False

# An Exit State is a dictionary of `keys` which represent the allowed EXIT STATE and check function (because number parsing has some edge cases).
EXIT_STATES = {
    WHOLE: None,
    E_NUMBER: lambda col: (WHOLE in col) or (FRACTIONAL in col),
    DOT: lambda col: len(col) > 1 and col[-2] not in [START, SIGN],
    FRACTIONAL: None,
}

def transition(c, available_states):
    """ Operation that gets the next state from the current character. Returns None if we cannot transition to the next state.
    """
    for state in available_states:
        if c in state:
            # print "From %s, transitioning into %s" % (c, state)
            return state

    # print "From %s, transitioning into None" % c
    return None

class Solution:
    # @param s, a string
    # @return a boolean
    def isNumber(self, s):
        previous = []
        state = START
        for c in s:
            if state is START:
                state = transition(c, [START, WHOLE, SIGN, DOT])
            elif state is WHOLE:
                state = transition(c, [WHOLE, DOT, E, END])
            elif state is DOT:
                state = transition(c, [FRACTIONAL, END, E])
            elif state is FRACTIONAL:
                state = transition(c, [FRACTIONAL, END, E])
            elif state is END:
                state = transition(c, [END])
            elif state is E:
                state = transition(c, [E_NUMBER, E_SIGN])
            elif state is E_NUMBER:
                state = transition(c, [E_NUMBER, END])
            elif state is E_SIGN:
                state = transition(c, [E_NUMBER, END])
            elif state is SIGN:
                state = transition(c, [DOT, E, WHOLE, FRACTIONAL])
            else:
                return False

            if state is not END:
                previous.append(state)

        last_state = previous[-1]

        if DEBUG:
            print "previous: %s" % previous
        for exit_state, fn in EXIT_STATES.items():
            if last_state is exit_state:
                return (fn == None) or fn(previous)

        return False


# This below section contains the unit tests I failed -- so I made sure to get them correct. Some of these are due to lack of requirements.
soln = Solution()
DEBUG = True

assert soln.isNumber(" ") == False
assert soln.isNumber("e") == False
assert soln.isNumber("123") == True
assert soln.isNumber("1 ") == True
assert soln.isNumber("2e10") == True
assert soln.isNumber(".1") == True
assert soln.isNumber("3.") == True
assert soln.isNumber(".") == False
assert soln.isNumber(" .") == False
assert soln.isNumber("3. ") == True
assert soln.isNumber(". ") == False
assert soln.isNumber("-1.") == True
assert soln.isNumber("+.8") == True
assert soln.isNumber(" -.") == False
assert soln.isNumber("46.e3") == True
assert soln.isNumber(".e1") == False
assert soln.isNumber(".2e81") == True
assert soln.isNumber(" 005047e+6") == True