8. String to Integer (atoi)

mplement 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++

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const char *

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spoilers alert... click to show requirements for atoi.

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.
2015年6月4日
The
atoi function first discards as many whitespace characters as necessary until the firstnon-whitespace character is found. Then, starting from this character, takes
an optionalinitial plus or minus sign followed by as many numerical digits as possible, and interpretsthem 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 instris
not a valid integral number, orif no such sequence exists because eitherstris empty or it contains only whitespacecharacters, no conversion
is performed.If no valid conversion could be performed, a zero value is returned. If the correct value isout of the range of representable values, the maximum integer value (2147483647) or theminimum
integer value (–2147483648) is returned.

首先要丢弃开头的空格。
可选的正负号。
数字。
再往后忽略。
如果第一个非空字符不是正负号或者数字，不进行转换，直接返回0。
超过INT_MAX或INT_MIN，直接返回相应最大值或最小值。
Solution:The
heart of this problem is dealing with overflow. A direct approach is to store the number as a string so we can evaluate at each step if the number had indeed overflowed.There
are some other ways to detect overflow that requires knowledge about how a specific programming language or operating system works.A
desirable solution does not require any assumption on how the language works. In each step we are appending a digit to the number by doing a multiplication and addition.
If the current number is greater than 214748364（INT_MAX/10）, we know it is going to overflow. On the other hand,
if the current number is equal to 214748364, we know that it will overflow only when the current digit is greater than or equal to 8. Remember to also consider edge case for
the smallest number, –2147483648 (–231).

class Solution {
public:
    int myAtoi(string str) {
        int i = 0;
        int len = str.length();
        while(i<len && str[i] == ' ')
            ++i;
        int sign = 1;
        if(i < len && str[i] == '+')
        {
            ++i;
        }
        else if(i < len && str[i] == '-')
        {
            sign = -1;
            ++i;
        }
        
        int MaxDiv10 = INT_MAX/10;
        int ret = 0;
        while(i < len && isdigit(str[i]))
        {
            int digit = str[i++] - '0';
            //overflow
            if(ret > MaxDiv10 || (ret == MaxDiv10 && digit >=8))
                return sign == 1?INT_MAX:INT_MIN;
            ret = ret *10 + digit;
        }
        
        return sign*ret;
        
    }
};