67. Add Binary

/*
https://leetcode.com/problems/add-binary/description/

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

*/

public class Solution {
    private static final Logger logger = LogManager.getLogger(Solution.class);
    private boolean previousSum = false;

    public String addBinary(String a, String b) {
        StringBuilder sum = new StringBuilder("");
        int lengthA = a.length();
        int lengthB = b.length();

        if (lengthA > lengthB) {
            for (int i = 1; i <= lengthB; i++) {
                if (previousSum) {
                    sum.insert(0, addThreeBinaryDigits(a.charAt(lengthA - i), b.charAt(lengthB - i)));
                } else {
                    sum.insert(0, addTwoBinaryDigits(a.charAt(lengthA - i), b.charAt(lengthB - i)));
                }
            }
            for (int i = lengthB + 1; i <= lengthA; i++) {
                if (previousSum) {
                    sum.insert(0, addTwoBinaryDigits(a.charAt(lengthA - i), '1'));
                } else {
                    sum.insert(0, a.charAt(lengthA - i));
                }
            }
            if (previousSum) {
                sum.insert(0, "1");
            }
        } else {
            for (int i = 1; i <= lengthA; i++) {
                if (previousSum) {
                    sum.insert(0, addThreeBinaryDigits(a.charAt(lengthA - i), b.charAt(lengthB - i)));
                } else {
                    sum.insert(0, addTwoBinaryDigits(a.charAt(lengthA - i), b.charAt(lengthB - i)));
                }
            }
            for (int i = lengthA + 1; i <= lengthB; i++) {
                if (previousSum) {
                    sum.insert(0, addTwoBinaryDigits(b.charAt(lengthB - i), '1'));
                } else {
                    sum.insert(0, b.charAt(lengthB - i));
                }
            }
            if (previousSum) {
                sum.insert(0, "1");
            }
        }
        return sum.toString();
    }

    private String addTwoBinaryDigits(char a, char b) {
        int sum = a + b - 96;
        switch (sum) {
            case 0:
                previousSum = false;
                return "0";
            case 1:
                previousSum = false;
                return "1";
            case 2:
                previousSum = true;
                return "0";
            default:
                return "Invalid input";
        }
    }

    private String addThreeBinaryDigits(char a, char b) {
        int sum = a + b - 95;
        switch (sum) {
            case 0:
                previousSum = false;
                return "0";
            case 1:
                previousSum = false;
                return "1";
            case 2:
                previousSum = true;
                return "0";
            case 3:
                previousSum = true;
                return "1";
            default:
                return "Invalid input";
        }
    }
}