Interview Question String to Integer (atoi)

By Fatih KABAKCI

The problem is to convert a String to a Integer by considering all possible input elements to give correct output to the user. The steps have been explained in the Java program.

Problem #8 - String to Integer (atoi).

Description: Implement atoi to convert a string to an integer.

Difficulty: Easy

Solution in Java

package com.fatihkabakci.Easy.Atoi;

/**
 * 
 * @author fkabakci
 * Problem Description: Implement atoi to convert a string to an integer.
 * 
 * Examples:
 * 
 * Example1: String x = "+123", return 123 as Integer
 * 
 * Example2: String x = "-123", return -123 as Integer
 * 
 * Example3: String x = "45.23,45", return 45 as Integer
 * 
 * Example4: String x = "2147483649", return 2147483647 as Integer
 * 
 * Solution:
 * 1. Ignore whitespace characters
 * 
 * 2. Check it if it has no character
 * 		if there is no, return 0
 * 
 * 3. Check it if it starts with "+" or "-" operator
 * 		if so, remove the first character
 * 
 * 4. Check it how many comma or dot it has,
 * 		if more than 1, input invalid then return 0,
 * 		otherwise cut by starting from either dot or comma operator to the rest of the string.
 * 
 * 5. Check it if it has a special character
 * 		if so, cut by starting from the special character to the rest of the string.
 * 
 * 6. Check the boundaries and see that how many digits there are,
 * 		if more than 10, then return either INT_MAX or INT_MIN depending on whether the sign is positive or negative
 * 
 * 7. Get each character then figure out its ASCII values. Then convert it to the integer by subtracting ASCII 0.
 * 		if overflow comes up, then fix the value by decreasing the total by distance from INT MAX.
 * 
 * 8. return total.
 */
public class StringToInteger {

	private int asciiToDigit(char c) {
		return c - "0";
	}
	private int frequency(String str, char c) {
		int counter = 0;
		for(int i = 0; i < str.length(); i++) {
			if(str.charAt(i) == c)
				counter++;
		}
		return counter;
	}
	
	private int hasSpecialCharacter(String str) {
		for(int i = 0; i < str.length(); i++) {
			if(!Character.isDigit(str.charAt(i)))
				return i;
		}
		return -1;
	}
	
	public int myAtoi(String str) {
		str = str.trim();
		
		if(str.isEmpty())
			return 0;
		
		// for + or -
		short sign = 1;
		char first = str.charAt(0);
		if (first == "+") {
			sign = 1;
			str = str.substring(1);
		}
		else if (first == "-") {
			sign = -1;
			str = str.substring(1);
		}
		
		// for commas
		int commaCounter = frequency(str, ",");
		if(commaCounter > 1)
			return 0;
		
		int commaIndex = str.indexOf(",");
		if (commaIndex != -1)
			str = str.substring(0, commaIndex);
		
		// for dots
		int dotCounter = frequency(str, ".");
		if(dotCounter > 1)
			return 0;
		
		int dotIndex = str.indexOf(".");
		if (dotIndex != -1)
			str = str.substring(0, dotIndex);
		
		// for specials
		int specialCharacterIndex = hasSpecialCharacter(str);
		if(specialCharacterIndex != -1)
			str = str.substring(0, specialCharacterIndex);
		
		// conversion
		
		int order = str.length();
		// boundaries
		if (order > 10)
			if (sign > 0)
				return Integer.MAX_VALUE;
			else
				return Integer.MIN_VALUE;
		
		int sum = 0;
		for (int i = 0; i < order; i++) {
			char ascii = str.charAt(i);
			int digit = asciiToDigit(ascii);
			for (int k = 1; k < order - i; k++) {
				digit = digit * 10;
			}
			sum = sum + digit;
			// overflow
			if (sum < 0) {
				if (sign == -1)
					sum = sum - (sum - 1 - Integer.MAX_VALUE);
				else
					sum = sum - (sum - Integer.MAX_VALUE);
			}
		}
		return sign * sum;
	}

	public static void main(String[] args) {
		StringToInteger converter = new StringToInteger();
		System.out.println(converter.myAtoi(""));
		System.out.println(converter.myAtoi(" "));
		System.out.println(converter.myAtoi("123"));
		System.out.println(converter.myAtoi("+123"));
		System.out.println(converter.myAtoi("-123"));
		System.out.println(converter.myAtoi("45.23"));
		System.out.println(converter.myAtoi("45.23,45"));
		System.out.println(converter.myAtoi("24,01"));
		System.out.println(converter.myAtoi("" + Integer.MAX_VALUE));
		System.out.println(converter.myAtoi("abc"));
		System.out.println(converter.myAtoi("3*2"));
		System.out.println(converter.myAtoi("010"));
		System.out.println(converter.myAtoi("  -0012a42"));
		System.out.println(converter.myAtoi("-2147483647")); // -2147483647
		System.out.println(converter.myAtoi("2147483647")); // 2147483647
		System.out.println(converter.myAtoi("2147483649")); // 2147483647
		System.out.println(converter.myAtoi("-2147483648")); // -2147483648
		System.out.println(converter.myAtoi("-2147483649")); // -2147483648
		System.out.println(converter.myAtoi("10522545459")); // 2147483647
		System.out.println(converter.myAtoi("-10522545459")); // -2147483648
	}
}

Output order:

1. 0
2. 0
3. 123
4. 123
5. -123
6. 45
7. 45
8. 24
9. 2147483647
10. 0
11. 3
12. 10
13. -12
14. -2147483647
15. 2147483647
16. 2147483647
17. -2147483648
18. -2147483648
19. 2147483647
20. -2147483648