008 String to Integer (atoi)

Original Description: converts a string to an integer. First, discards as many whitespace characters as necessary until the first non-whitespace character. 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. Return 0.

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• 要求： 给一个string,将其转化为interger.
  1. 若string起始为 空格 ，丢弃 空格 直到非空格字符；
  2. 整数之后的其他字符忽略
  3. 不以 数字，正负号，或 空格 起始的字符，不转换。返回0。
  4. 保留数字的 正负号。
  5. 转换后的数字范围应在 \([−2^{31}, 2^{31} − 1]\), 否则返回边界。

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• 返回： string对应的 integer

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• 例子：
  • input: “ -42”
  • output: -42
  • input: “words and 987”
  • output: 0

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Solution

• 将string split（去除空格）到list \(l\), 如果\(l[0]\) 为数字，判断边界 \(min((2**31)-1,int(l[0]))\)，返回。
• 否则， 读取正负号， 之后若第一位非数字，返回0,否则逐位读取string的字符，直到非数字。判断边界。返回值。

class Solution(object):
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        if len(str)==0:
            return 0
        else:
            l = str.split()
            if l == []:
                return 0
            elif len(l[0]) == 1 and not l[0].isdigit():
                return 0
            elif l[0].isdigit():
                return min((2**31)-1,int(l[0]))
            else:
                out = self.getnumber(l[0])
                return out
    def getnumber(self, str):
        if str[0] == '-':
            sign = -1
            i = 1
        elif str[0] == '+':
            sign = 1
            i = 1
        else: 
            sign = 1
            i = 0
        out = ""
        if not str[i].isdigit():
            return 0
        else:
            while str[i].isdigit():
                out += str[i]
                i += 1
                if i >= len(str):
                    break
            if sign*int(out) <0 :
                return max(-1*(2**31), sign*int(out))
            else:
                return min((2**31)-1, sign*int(out))