Leetcode: String to Integer (atoi)

#define INT_MAX 2147483647
#define INT_MIN (-INT_MAX-1)
class Solution 
{
public:
    int atoi(const char *str)
    {
    	if (str == nullptr)
    	{
    		return 0;
    	}
    	int position = 0;
    	while (str[position] == ' ')
    	{
    		position++;
    	}
    	if (str[position] == '\0')
    	{
    		return 0;
    	}
    
    	int signal = 1;
    	if (str[position] == '+' || str[position] == '-')
    	{
    		if (str[position] == '-')
    		{
    			signal = -1;
    		}
    		position++;
    	}
    
    	long long result = 0;// 注意这里是long long
    	while (str[position] >= '0' && str[position] <= '9')
    	{
    		result = result * 10 + str[position] - '0';
    		if (result * signal > INT_MAX)
    		{
    			return INT_MAX;
    		}
    		if (result * signal < INT_MIN)
    		{
    			return INT_MIN;
    		}
    		
    		position++;
    	}
    	return result * signal;
    }
};