myAtoi.cpp

#include <cctype>
#include <string>
#include <limits>

class Solution {
public:
    int myAtoi(string s) {
        char sign{};
        constexpr int high{ std::numeric_limits<int>::max() };
        constexpr int low{ std::numeric_limits<int>::min() };

        std::size_t index{};
        if (!isdigit(s[0]) && s[0] != '+' && s[0] != '-')
        {
            do
            {
                if (!isspace(s[index]) && s[index] != '+' && s[index] != '-')
                {
                    return 0;
                }
                ++index;
                
            } while (!isdigit(s[index]) && s[index] != '+' && s[index] != '-');
        }

        if ((s[index] == '+' || s[index] == '-') && (index + 1) < s.length())
        {
            if (s[index + 1] == '-' || s[index + 1] == '+')
            {
                return 0;
            }
            else
            {
                sign = s[index];
                ++index;
            }
        }

        int value{};
        for (std::size_t i{ index }, n{ s.length() }; i < n && isdigit(s[i]); ++i)
        {
            if (value > (high / 10) || value * 10 > high - (s[i] - '0'))
            {
                return sign == '-' ? low : high;
            }
            value = value * 10 + (s[i] - '0');
        }

        if (sign == '-')
        {
            value *= -1;
        }
        return value;
    }
};