第一題：String to Integer (atoi)

• 描述：

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.

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.

• Note:

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: [−2^31,  2^31 − 1]. If the numerical value is out of the range of representable values, INT_MAX (2^31 − 1) or INT_MIN (−2^31) is returned.

• Example

Example 1:

Input: "42"
Output: 42

Example 2:

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.

Example 3:

Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.

Example 4:

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 (−2^31) is returned.

• process of thinking

看注意點可以發現，如果返回一個數字，則有以下幾種情況：字串中有數字，且出現的第一個數字前面不能有除了（'  ','+','-'）的其他字元，再注意最值處理即可。

• 演算法程式碼一

class Solution:
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        m = re.match("\s*([-\+]?[\d]+).*", str)
        if not m:
            return 0
        else:
            x = int(m.group(1))
            if x < -2**31:
                return -2**31
            elif x > 2**31 - 1:
                return 2**31 - 1
            else:
                return x

•  演算法程式碼二

class Solution(object):
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        lower_bound = -2**31
        upper_bound = 2**31-1
        nums = str.split()
        if not nums:return 0
        tmp = ""
        avai = set(['0','1','2','3','4','5','6','7','8','9'])
        flag = False
        for i,c in enumerate(nums[0]):
            if i == 0:
                if c in avai or c == '-' or c == '+':
                    tmp += c
                else:
                    break
            else:
                if c in avai:
                    tmp += c
                else:
                    break
        try:
            num = int(tmp)
        except:
            return 0
        
        if num < lower_bound:return lower_bound
        if num > upper_bound:return upper_bound
        return num