Add Binary

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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        StringBuffer sb = new StringBuffer();
        String l = null;
        String s = null;
        if(a == null && b == null) return null;
        else if(a == "" && b == "") return "";
        else if(a == "" && b.length() > 0) return b;
        else if(a.length() > 0 && b == "") return a;
        if(a.length() > b.length()){
            l = a;
            s = b;
        }else{
            l = b;
            s = a;
        }
        int j = s.length() - 1;
        int carry = 0;
        for(int i = l.length() - 1; i > -1; i --){
            char c = '0';
            if(j > -1){
                int aa = l.charAt(i) - '0';
                int bb = s.charAt(j --) - '0';
                c += (aa + bb + carry) % 2;
                carry = (aa + bb + carry) / 2;
            }else{
                int aa = l.charAt(i) - '0';
                c += (aa + carry) % 2;
                carry = (aa + carry) / 2;
            }
            sb.insert(0,c);
        }
        if(carry == 1)
            sb.insert(0,"1");
        return sb.toString();
    }
}

 第二遍：

使用位运算： ^ 异或.

public class Solution {
    public String addBinary(String a, String b) {
        // Start typing your Java solution below
        // DO NOT write main() function
        StringBuffer result = new StringBuffer();
        int carry = 0;
        for(int i = 1; i <= a.length() || i <= b.length(); i ++){
            int ac = 0, bc = 0;
            if(i <= a.length()) ac = a.charAt(a.length() - i) - '0';
            if(i <= b.length()) bc = b.charAt(b.length() - i) - '0';
            result.insert(0, ac ^ bc ^ carry);
            carry = (ac + bc + carry) > 1 ? 1 : 0;
        }
        if(carry > 0) result.insert(0, carry);
        return result.toString();
    }
}