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.

这题主要考察的是字符的操作。首先检查第一位是否为正负号，然后再依次取数值。

class Solution:
    # @return an integer
    def atoi(self, str):
        if len(str)==0:
            return 0
        str=str.split()
        s=str[0]
        i=0
        flag=0
        if s[0]=='-' or s[0]=='+':
            i+=1
            if s[0]=='-':
                flag=1
        y=0
        while i<len(s):
            if ord(s[i])<ord('0') or ord(s[i])>ord('9'):
                break
            else:
                y=y*10+ord(s[i])-ord('0')
            i+=1
        if flag==1:
            y=-y
        if y<-2147483648:
            return -2147483648
        elif y>2147483647:
            return 2147483647
        else:
            return y