valid-number.py

# Time:  O(n)
# Space: O(1)

class InputType(object):
    INVALID    = 0
    SPACE      = 1
    SIGN       = 2
    DIGIT      = 3
    DOT        = 4
    EXPONENT   = 5


# regular expression: "^\s*[\+-]?((\d+(\.\d*)?)|\.\d+)([eE][\+-]?\d+)?\s*$"
# automata: http://images.cnitblog.com/i/627993/201405/012016243309923.png
class Solution(object):
    def isNumber(self, s):
        """
        :type s: str
        :rtype: bool
        """
        transition_table = [[-1,  0,  3,  1,  2, -1],     # next states for state 0
                            [-1,  8, -1,  1,  4,  5],     # next states for state 1
                            [-1, -1, -1,  4, -1, -1],     # next states for state 2
                            [-1, -1, -1,  1,  2, -1],     # next states for state 3
                            [-1,  8, -1,  4, -1,  5],     # next states for state 4
                            [-1, -1,  6,  7, -1, -1],     # next states for state 5
                            [-1, -1, -1,  7, -1, -1],     # next states for state 6
                            [-1,  8, -1,  7, -1, -1],     # next states for state 7
                            [-1,  8, -1, -1, -1, -1]]     # next states for state 8

        state = 0
        for char in s:
            inputType = InputType.INVALID
            if char.isspace():
                inputType = InputType.SPACE
            elif char == '+' or char == '-':
                inputType = InputType.SIGN
            elif char.isdigit():
                inputType = InputType.DIGIT
            elif char == '.':
                inputType = InputType.DOT
            elif char == 'e' or char == 'E':
                inputType = InputType.EXPONENT

            state = transition_table[state][inputType]

            if state == -1:
                return False

        return state == 1 or state == 4 or state == 7 or state == 8


class Solution2(object):
    def isNumber(self, s):
        """
        :type s: str
        :rtype: bool
        """
        import re
        return bool(re.match("^\s*[\+-]?((\d+(\.\d*)?)|\.\d+)([eE][\+-]?\d+)?\s*$", s))