LeetCode题解：String to Integer (atoi)

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.

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.

思路：

要小心INT_MAX和INT_MIN附近的值。特别注意INT_MIN的绝对值比INT_MAX大1。

题解：

class Solution {
public:
    const int MAXINT_DIV10 = INT_MAX / 10;
    const int MAXINT_MOD10 = INT_MAX % 10;
    const int MININT_DIV10 = -(INT_MIN / 10);
    const int MININT_MOD10 = -(INT_MIN % 10);
    
    int atoi (const char* str)
    {
        int sgn = 1;
        int result = 0;
        
        /* skip spaces */
        while (str != NULL && isspace (*str)) ++str;
        
        /* check starting character */
        if (str == NULL || (!isdigit (*str) && *str != '+' && *str != '-'))
            return 0;
            
        /* read the flag */
        if (*str == '-')
            sgn = -1, ++str;
        else if (*str == '+')
            sgn = 1, ++str;
            
        while (isdigit (*str))
        {
            int val = *str - '0';
            
            // check overflow
            if (sgn == 1)
            {
                if ( (result > MAXINT_DIV10) ||
                        (result == MAXINT_DIV10 && val > MAXINT_MOD10))
                    return INT_MAX;
            }
            else
            {
                if (result > MININT_DIV10)
                    return INT_MIN;
                else if (result == MININT_DIV10 && val >= MININT_MOD10)
                    return INT_MIN;
            }
    
            result *= 10;
            result += val;
    
            ++str;
        }
    
        return result * sgn;
    }
};