String to Integer (atoi) ***

Implement 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++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button  to reset your code definition.

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

分析：好烦的一道题。自己写了好多次都没AC，果然群众的力量比较大。12ms

class Solution {
public:
    int myAtoi(string str) {
        long long result = 0;
        int sign = 1;
        int size = str.length();
        int index = 0;
        
        while(str[index] == ' ' && index < size) index++;
        
        if(str[index] == '+') index++;
        else if(str[index] == '-'){
            index++;
            sign = -1;
        }
        
        for(; index < size; index++){
            if(str[index] < '0' || str[index] > '9') break;
            
            result = result * 10 + str[index] - '0';
            if(result > INT_MAX) return sign == 1 ? INT_MAX : INT_MIN;
        }
        return (int) sign * result;
    }
};