0067-Add-Binary.java

class Solution {
    public String addBinary(String a, String b) {
        // Go from right to left
        int len1 = a.length();
        int len2 = b.length();

        int minLen = Math.min(len1, len2);
        int maxLen = Math.max(len1, len2);

        int index1 = len1 - 1;
        int index2 = len2 - 1;

        StringBuilder bld = new StringBuilder();
        boolean carry = false;

        while((index1 >= 0) || (index2 >= 0) || carry) {
            char c1 = (index1 >= 0) ? a.charAt(index1) : '0';
            char c2 = (index2 >= 0) ? b.charAt(index2) : '0';

            char sum;
            if((c1 == '0') && (c2 == '0')) {        // Both zeros
                sum = carry ? '1' : '0';
                carry = false;
            }
            else if((c1 == '1') && (c2 == '1')) {   // Both ones
                sum = carry ? '1' : '0';
                carry = true;
            }
            else {                                  // Any one with a one
                sum = carry ? '0' : '1';
                // carry will be forwarded
            }
            bld.append(sum);

            index1--;
            index2--;
        }

        return bld.reverse().toString();
    }
}