String to Integer (atoi) Cont…(Faster)

Question: 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.

You can view the full question here.

If this is your first time checking out this question, you may want to take a look at the posts here and here to start off with.

Approach 3: After evaluating the previous two approaches, we can say that the string operations such as charAt() and trim() turn out to be very expensive. So, we have simply tried to eliminate these method usages and have resorted to a more light weight approach by converting the string to a character array and then performing iterative checks.

//Approach 3
//Runtime: 1ms
//Memory usage: 37.1MB
class Solution {
    public int myAtoi(String str) {
        int index = 0;
  int length = str.length();
        char[] array = str.toCharArray();
        while(index<length && array[index]==' ')
        {
            index++;
        }
        
        int sign = 1;
        if(index<length && array[index]=='-'){
            index++;
            sign = -1;
        } else if(index<length && array[index]=='+'){
            index++;
        }
        double finalInt = 0;
        for(; index<length; index++){
            if(array[index]>='0' && array[index]<='9'){
                finalInt=finalInt*10 + array[index]-'0';
            } else {
                break;
            }            
        }
        if(finalInt>Integer.MAX_VALUE){
            return sign==1?Integer.MAX_VALUE:Integer.MIN_VALUE;
        } else {
            return sign * (int)finalInt;
        }
    }
}

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Cheers & Chao!