【Leet Code】String to Integer (atoi) ——常考類型題

String to Integer (atoi)

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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.

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

字符串的操作，寫程序經常性遇到，string這個類真的非常有用喲，題目要求自行實現atoi的功能：

class Solution 
{
public:
    int atoi(const char *str) 
    {
        while(' ' == *str)
        {
            str++;
        }
        bool isNegative = false;
        if('-' == *str) 
        {
            isNegative = true;
            str++;
        } 
        else if('+' == *str) 
        {
            str++;
        }
        long long ret = 0;
        while(*str) 
        {
            if( isdigit(*str) ) 
            {
                ret = ret * 10 + (*str - '0');
                if(isNegative && (-ret <= INT_MIN))
                {
                    return INT_MIN;
                }
                if(!isNegative && (ret >= INT_MAX))
                {
                    return INT_MAX;
                }
            } 
            else 
            {
                break;
            }
            str++;
        }
        return (isNegative ? -ret : ret);
    }
};