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

For example,

a = "11"

b = "1"

Return "100".

思路：二进制加法，比較简单。代码例如以下：

public class Solution {

    public String addBinary(String a, String b) {

        int len = Math.max(a.length(),b.length());

        char[] ch = new char[len + 1];//预防进位

        int i = a.length() - 1;

        int j = b.length() - 1;

        //不考虑进位，所有相加

        while(i >= 0 && j >= 0){

            ch[len--] = (char) (a.charAt(i--) + b.charAt(j--) - '0');

        }

        //余下仅有一个还未加完

        while(i >= 0){

            ch[len--] = a.charAt(i--);

        }

        while(j >= 0){

            ch[len--] = b.charAt(j--);

        }

        int k = 0;//进位

        //处理进位

        for(i = ch.length-1; i >=1; i--){

            ch[i] = (char) (k + ch[i] - '0');

            k = ch[i]/2;

            ch[i] = (char) (ch[i]%2 + '0');

        }

        //推断是否还有进位

        ch[0] = (char) (k > 0 ?

k +'0': '0');

        //消除开头的0

        return new String(ch).replaceAll("^0", "");

    }

}