string-to-integer-atoi.py

# -*- coding:utf-8 -*-


# Implement atoi which converts a string to an integer.
#
# The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.
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# The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.
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# If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.
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# If no valid conversion could be performed, a zero value is returned.
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# Note:
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#
# 	Only the space character ' ' is considered as whitespace character.
# 	Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231,  231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.
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#
# Example 1:
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# Input: "42"
# Output: 42
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#
# Example 2:
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# Input: "   -42"
# Output: -42
# Explanation: The first non-whitespace character is '-', which is the minus sign.
#              Then take as many numerical digits as possible, which gets 42.
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#
# Example 3:
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# Input: "4193 with words"
# Output: 4193
# Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
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# Example 4:
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# Input: "words and 987"
# Output: 0
# Explanation: The first non-whitespace character is 'w', which is not a numerical 
#              digit or a +/- sign. Therefore no valid conversion could be performed.
#
# Example 5:
#
#
# Input: "-91283472332"
# Output: -2147483648
# Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
#              Thefore INT_MIN (−231) is returned.
#


class Solution(object):
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        import re
        p = re.compile(r'^[+-]?[0-9]+')
        m = re.match(p, str.strip())
        if m:
            ret = int(m.group())
            if ret < -0x80000000:
                return -0x80000000
            elif ret > 0x7fffffff:
                return 0x7fffffff
            return ret
        else:
            return 0