AddBinary.java

import java.util.Scanner;

public class AddBinary {
    public static void main(String... args) {
        Scanner scanner = new Scanner(System.in);
        String firstSt = scanner.nextLine();
        String secondSt = scanner.nextLine();
        String result = getSummation(firstSt, secondSt);
        System.out.println(result);
    }

    static String getSummation(String firstSt, String secondSt) {
        int s1 = firstSt.length();
        int s2 = secondSt.length();
        int carry = 0;
        int sum;
        StringBuilder result = new StringBuilder();
        while (s1>0 || s2 >0) {
            int firstCurrent = 0;
            int secondCurrent = 0;
            if (s1>0) {
                s1--;
                firstCurrent = firstSt.charAt(s1) - '0';
            }
            if (s2>0) {
                s2--;
                secondCurrent = secondSt.charAt(s2) - '0';
            }
            sum = carry + firstCurrent + secondCurrent;
            if (sum == 3) {
                result.insert(0, "1");
                carry = 1;
            } else if (sum == 2) {
                result.insert(0, "0");
                carry = 1;
            } else if (sum == 1) {
                result.insert(0, "1");
                carry = 0;
            } else if (sum == 0) {
                result.insert(0, "0");
                carry = 0;
            }
        }
        if (carry ==1) result.insert(0, "1");
        return result.toString();
    }
}