AddBinary.java

package string;

/**
 * Created by gouthamvidyapradhan on 25/11/2017.
 *
 * Given two binary strings, return their sum (also a binary string).

 For example,
 a = "11"
 b = "1"
 Return "100".
 */
public class AddBinary {

    /**
     * Main method
     * @param args
     */
    public static void main(String[] args) {
        System.out.println(new AddBinary().addBinary("000001010000101001", "0"));
    }

    public String addBinary(String a, String b) {
        if(a.length() > b.length()){
            return calculate(a, b);
        } else return calculate(b, a);
    }

    /**
     * Calculate sum
     * @param a length of a should always be greater or equal to b
     * @param b length of b should always be smaller of equal to a
     * @return
     */
    private String calculate(String a, String b){
        int carry = 0;
        int d = a.length() - b.length();
        StringBuilder sb = new StringBuilder();
        for(int i = a.length() - 1; i >= 0; i --){
            int first = Integer.parseInt(String.valueOf(a.charAt(i)));
            int second = i - d >= 0 ? Integer.parseInt(String.valueOf(b.charAt(i - d))) : 0;
            int sum = (first + second + carry);
            carry = sum / 2;
            sb.append(sum % 2);
        }
        if(carry != 0)
            sb.append(carry);
        return sb.reverse().toString();
    }
}