leetcode--Add Binary

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

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

public class Solution {
    /**The program is used to calculate the sum of two binaries. This is a fundamental problem.<br> 
     * @param a --binary string as the first operand
     * @param b --binary string as the second operand
     * @return a string --the sum of binary a and b
     * @author Averill Zheng
     * @version 2014-06-03
     * @since JDK 1.7
     */
    public String addBinary(String a, String b) {
        int lengthOfA = a.length(), lengthOfB = b.length();
		StringBuffer resultBuffer = new StringBuffer();
		int carry = 0;
		while(lengthOfA > 0 && lengthOfB > 0){
			int sum = (a.charAt(lengthOfA - 1) -'0') + (b.charAt(lengthOfB - 1)- '0') + carry;
			carry = sum / 2;
			resultBuffer.insert(0, sum % 2);
			--lengthOfA;
			--lengthOfB;
		}
		String remaining = (lengthOfA == 0)? b : a;
		int index = (lengthOfA == 0) ? lengthOfB : lengthOfA;
		while(index > 0){
			int sum = (remaining.charAt(index - 1) -'0') + carry;
			carry = sum / 2;
			resultBuffer.insert(0, sum % 2);
			--index;
		}
		if(carry != 0)
			resultBuffer.insert(0, 1);
		return resultBuffer.toString();        
    }
}