[Leetcode] 8. String to Integer (atoi)

Problem：

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

Update (2015-02-10):
The signature of the C++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button to reset your code definition.

Requirements for atoi:
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.

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.

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.

If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.

思路：
注意这里输入的string可以有若干个空格，以及有可能有多于一个的“+”或“-”，也有可能有除了数字以及加减号其他的字符。另外还要处理整数越界的情况。

Solution：

# -*- coding: utf-8 -*-
"""
Created on Thu Jan 19 12:14:36 2017

@author: liangsht
"""

class Solution(object):
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        integer = 0
        start = False
        tagFlag = False
        if len(str) == 0:
            return 0
        for i in xrange(0,len(str)):
            if str[i] >= '0' and str[i] <= '9' :
                integer = integer * 10 + int(int(str[i])-int('0'))
                start = True
                #print integer
            elif (str[i] == '-' or str[i] == '+') and start == False:
                start = True
                tagFlag = True
                tagpos = i
            elif str[i] == ' ' and start == False:
                continue
            else:
                break
        if tagFlag == True:
            integer = -integer if str[tagpos] == '-' else integer
        integer = 2147483647 if integer > 2147483647 else integer
        integer = -2147483648 if integer < -2147483648 else integer
        return integer