atoi.java

/**
 * 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.
 * 
 * Keys: Whitespaces, Additional chars, Signs, Out of range
 * 
 * Requirements for atoi:
 * 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. If the
 * correct value is out of the range of representable values, INT_MAX
 * (2147483647) or INT_MIN (-2147483648) is returned.
 * 
 * Tags: Math, String
 */
class atoi {
    
    public static void main(String[] args) {
        System.out.println("MAX_VALUE: " + Integer.MAX_VALUE);
        System.out.println("MIN_VALUE: " + Integer.MIN_VALUE);
        System.out.println("2147483648: " + atoi("2147483648"));
        System.out.println("-2147483647: " + atoi("-2147483647"));
        System.out.println("-2147483648: " + atoi("-2147483648"));
    }
    
    /**
     * Whitespace, sign, out of range
     * Trim the unnecessary whitespaces
     * initialize a variable as long to store the result
     * use a boolean as a flag to mark whether its negative
     * return MAX or MIN if its out of range
     */
    public static int atoi(String str) {
        /*validate input*/
        if (str == null || str.length() == 0) return 0;
        long longRes = 0; // result can be out of range
        /*whitespaces*/
        str = str.trim(); // remove front and trailing whitespaces
        /*sign*/
        boolean neg = false; // is negative or not
        if (str.charAt(0) == '-') {
            neg = true;
            str = str.substring(1, str.length());
        } else if (str.charAt(0) == '+') {
            str = str.substring(1, str.length());
        }
        /*calculation*/
        int i = 0;
        while (i < str.length()) { // calculate without sign
            char c = str.charAt(i);
            if (c >= '0' && c <= '9') {
                longRes = longRes * 10 + (c - '0');
            } else break; // break when not a digit
            i++;
        }
        longRes = neg ? longRes * (-1) : longRes; // add sign
        /*out of range*/
        if (longRes > Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        } else if (longRes < Integer.MIN_VALUE) {
            return Integer.MIN_VALUE;
        }
        
        return (int)longRes;
    }
    
    /**
     * To deal with overflow, inspect the current number before multiplication. 
     * If the current number is greater than 214748364, we know it is going to
     * overflow. On the other hand, if the current number is equal to 
     * 214748364, we know that it will overflow only when the current digit is
     * greater than or equal to 8.
     */
    private static final int maxDiv10 = Integer.MAX_VALUE / 10;

    public int atoi(String str) {
        int n = str.length();
        int i = 0;
        while (i < n && Character.isWhitespace(str.charAt(i))) i++;
        int sign = 1;
        if (i < n && str.charAt(i) == '+') {
            i++;
        } else if (i < n && str.charAt(i) == '-') {
            sign = -1;
            i++;
        }
        int num = 0;
        while (i < n && Character.isDigit(str.charAt(i))) {
            int digit = Character.getNumericValue(str.charAt(i));
            if (num > maxDiv10 || num == maxDiv10 && digit >= 8) {
                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            }
            num = num * 10 + digit;
            i++;
        }  
        return sign * num;
    }
}