67. Add Binary

题目：Given two binary strings, return their sum (also a binary string).
The input strings are both non-empty and contains only characters 1 or 0.
Example 1:
Input: a = "11", b = "1"
Output: "100"
Example 2:
Input: a = "1010", b = "1011"
Output: "10101''

意思是:输入两个二进制字符串，输出两个二进制相加后的结果的字符串。
思路是:两个二进制字符串从最后开始遍历，拿到每个char，可能是'1'或'0'，两个字符串的char相互累加，由于'1'在ASCII对应为49，'0'为48，所以累加之前要-'0'，最后，两个char计算结果可能是0到2，所以sum%2为当前的值，如果是2，还需要进一位，所以sum/2，然后在下次循环中加上。

class Solution {
    public String addBinary(String a, String b) {
        
        int aLen = a.length()-1;
        int bLen = b.length()-1;
        StringBuffer sb = new StringBuffer();
       
        int temp = 0;
        while(aLen >= 0 || bLen >= 0){
             int sum = temp;
            if(aLen >= 0){
                sum += a.charAt(aLen) - '0';
                aLen--;
            }
            
            if(bLen >= 0){
                sum += b.charAt(bLen) - '0';
                bLen--;
            }
            sb.append(sum%2);
            temp = sum/2;
         
        }
        
         if(temp !=0 )            
            sb.append(1);
        
        return sb.reverse().toString();
    }
}