Java

package leetcode.template;

public class Atoi {


    public int myAtoi(String str) {
        int index = 0, sign = 1, total = 0;
        //1. Empty string   
        if (str.length() == 0) return 0;
        //2. Remove Spaces   
        while (str.charAt(index) == ' ' && index < str.length())
            index++;
        //3. Handle signs   
        if (str.charAt(index) == '+' || str.charAt(index) == '-') {
            sign = str.charAt(index) == '+' ? 1 : -1;
            index++;
        }
        //4. Convert number and avoid overflow 
        while (index < str.length()) {
            int digit = str.charAt(index) - '0';
            if (digit < 0 || digit > 9) break;

            if (Integer.MAX_VALUE / 10 < total || Integer.MAX_VALUE / 10 == total && Integer.MAX_VALUE % 10 < digit)
                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            total = 10 * total + digit;
            index++;
        }
        return total * sign;
    }
}

Python

class Atoi(object):

    def myAtoi(self, s):
        if len(s) == 0: return 0
        ls = list(s.strip())
        sign = -1 if ls[0] == '-' else 1
        if ls[0] in ['-', '+']: del ls[0]
        ret, i = 0, 0
        while i < len(ls) and ls[i].isdigit():
            ret = ret * 10 + ord(ls[i]) - ord('0')
            i += 1
        return max(-2 ** 31, min(sign * ret, 2 ** 31 - 1))