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();

}

}