Add Binary.java

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

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

public class Solution {
    public String addBinary(String a, String b) {
    	int i = a.length() - 1;
	int j = b.length() - 1;
	int da = 0;
	int db = 0;
	int adv = 0;
	StringBuffer result = new StringBuffer();
	while (i >= 0 && j >= 0) {
	    da = a.charAt(i--) == '0' ? 0 : 1;
	    db = b.charAt(j--) == '0' ? 0 : 1;
	    int d = da + db + adv;
	    result.append(d % 2 == 0 ? '0' : '1');
	    adv = d >> 1;
	}
	if (i >= 0) {
	    while (i >= 0) {
		da = a.charAt(i--) == '0' ? 0 : 1;
		int d = da + adv;
		result.append(d % 2 == 0 ? '0' : '1');
		adv = d >> 1;
	    }
	} else if (j >= 0) {
	    while (j >= 0) {
		db = b.charAt(j--) == '0' ? 0 : 1;
		int d = db + adv;
		result.append(d % 2 == 0 ? '0' : '1');
		adv = d >> 1;
	    }
	}
	if (adv == 1) {
	    result.append('1');
	}
	return result.reverse().toString();
    }
}