stringToIntegerATOI.java

Implement atoi which converts a string to an integer.

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.

Note:

Only the space character ' ' is considered as whitespace character.
Assume we are dealing with an environment which could only store integers within the 32-bit signed 
integer range: [−231,  231 − 1]. If the numerical value is out of the range of representable values, 
INT_MAX (231 − 1) or INT_MIN (−231) is returned.

Example:

Input: "42"
Output: 42


Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.



INTUITION, 4 steps!! remove white spaces, get sign, convert to number and check for overflow 

//TC: O(n) where n is length of input string 
//SC: O(1) only use local varialbes 
class Solution {
    public int myAtoi(String str) {
        int index = 0;
        int total = 0;
        int sign = 1;
        
        // Check if empty string
        if(str.length() == 0)
            return 0;
        
        // remove white spaces from the string
        while(index < str.length() && str.charAt(index) == ' ')
            index++;
        
        if (index == str.length()) return 0;
        
        // get the sign
        if(str.charAt(index) == '+' || str.charAt(index) == '-') {
            sign = str.charAt(index) == '+' ? 1 : -1;
            index++;
        }
        
        // convert to the actual number and make sure it's not overflow
        while(index < str.length()) {
            int digit = str.charAt(index) - '0';
            if(digit < 0 || digit > 9) break;
            
            // check for overflow
            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 = total*10 + digit;
            index++; // don't forget to increment the counter
        }
        return total*sign;
    }
}