[LeetCode] 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.

class Solution {
public:
    int myAtoi(string str) {
        if(str.empty()) return 0;
        int i = 0;
        bool hasPlus = false;
        while(i<str.length() && str[i] == ' ') i++;
        
        if(i<str.length() && str[i] == '+') {
            i++;
            hasPlus = true;
        }
        int flag = 1;
        if(!hasPlus && i < str.length() && str[i] == '-') 
        {
            flag = -1;
            i++;
        }

        long res = 0;
        
        while(i<str.length())
        {
            if(!isdigit(str[i]) ) return res*flag;
            res = res*10 + str[i] - '0';
            if(res > INT_MAX)
            {
                if(flag == 1) return INT_MAX;
                if(res != (long)-1*INT_MIN ) return INT_MIN;
            }
            i++;
        }
        
        return res * flag;
    }
};