Implement atoi to convert a string to an integer.

思路

atoi函数的主要功能就是将字符串转换成整数，题目给出了提示，需要我们考虑清楚所有可能的情况。

百密必有一疏，在我写完代码第一次运行的时候，还是漏掉了以下三项：

  1. 应当去掉尽可能多的空格；
  2. 字符串起始可以含有+号，并将此数视为正数；
  3. 在数字之后应当可以包含任何字符。
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class Solution(object):
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        
        symbols = ["+","-"]
        numbers = ["0","1","2","3","4","5","6","7","8","9"]
        length = len(str)
        symbol = "+"
        
        if (length == 0):
            return 0
        
        res = []
        cur = 0
        # 去除一开始的空格
        while (str[cur] == " "):
            cur += 1
        # 判断第一个字符是不是所期待的字符
        if ( (str[cur] not in symbols) and (str[cur] not in numbers) ):
            return 0
        else:
            # 是符号的话，就存起来；否则开始转换成数字
            if (str[cur] in symbols):
                symbol = str[cur]
            else:
                res.append(str[cur])
            cur += 1
            
            # 注意到数字之后还能有别的乱七八糟的符号
            for i in range(cur, length):
                if (str[i] not in numbers):
                    break
                else:
                    res.append(str[i])
        
        # 处理转换数字的操作
        reslen = len(res)
        if (reslen <= 0):
            return 0
        else:
            answer = 0
            count = 0
            for i in res[::-1]:
                answer += 10 ** count * int(i)
                count += 1
            if (symbol == "-"):
                answer = -answer
            # 注意数字不能越界，python的整数范围比int的值要大
            if (answer > 2147483647):
                answer = 2147483647
            elif (answer < -2147483648):
                answer = -2147483648
            return answer