65. Add Binary

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

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

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public class Solution {
    public String addBinary(String a, String b) {
      
        // convert to int
        if(a == null || b == null) return null;
        
        int x = 0;
        for(int i=0; i<a.length(); i++){
            x <<= 1;
            x |= a.charAt(i) - '0';
        }
        
        int y = 0;
        for(int i=0; i<b.length(); i++){
            y <<= 1;
            y |= b.charAt(i) - '0';
        }
        
        int sum = x + y;
      
        // !!! if sum == 0
        if(sum == 0)　　return "0";
        
        
        StringBuffer sb = new StringBuffer();    
        while(sum > 0){
            int val = sum & 0x1;
            sb.append(val);
            sum >>= 1;
        }
        
        sb.reverse();
        
        return sb.toString();
    }
    
}