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.

分析：

class Solution {
public:
    int myAtoi(string str) {

		if(str.length() == 0)
			return 0;
		//用于存储结果
		long long result = 0 ;
		int sign = 1 , i=0;

		while(str[i] == ' ')
		{
			if (str[i] == ' ')
				i++;
		}

		if(str[i] == '+')
			i++;
		else if(str[i] == '-')
		{
			sign = -1;
			i++;
		}
		
		for(int j=i ; j<str.length() ; j++)
		{
			if(str[j]>='0' && str[j]<='9')
			{
				result = result * 10 + (str[j]-'0');
				if(result > INT_MAX)
					return sign<0 ? INT_MIN : INT_MAX;
			}
			else
				break;
		}
		result *= sign;
		return (int)result;
    }
};