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 m = a.length(),n = b.length();
        String rec = "";
        int maxLen = Math.max(m,n);
        int carry = 0;
        for(int i=maxLen-1;i>=0;i--)
        {
            int p=0,q=0;
            if(m>0)
              p = a.charAt(--m)-'0';
            else
              p = 0;
            if(n>0)
              q = b.charAt(--n)-'0';
            else
              q = 0;
            int sum = p + q + carry;
            carry = sum/2;
            rec = sum%2 + rec;  //直接转换为String类型
        }
        return carry == 0 ? rec : "1"+rec;
    }
}