Add Binary

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

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

 

每次相加的时候每位会出现0，1 ，2 ，3的情况，其对应的二进制是0，1，0，1.所以采用%2的形式。

进位为0或者1，满2有进位，其余为0。所以采用/2.

对字符串的处理，因为是java会比较简单，强语言类型可以强制转换。显示的强制转换只是方便阅读代码，

所以最好不要依赖jdk实现数据类型的转换。

class Solution {
    public String addBinary(String a, String b) {
        int sum = 0;//结果
        String res = "" ;//字符串结果
        int carries = 0;//进位

        int al = a.length()-1;
        int bl = b.length()-1;
        
        //判断长度
        if(a.length() > b.length()){
            while(bl >= 0){
              sum =  (int)(a.charAt(al) - '0') + (int)(b.charAt(bl) - '0') + carries;
              res = String.valueOf(sum%2) + res;
              carries = sum/2;
              al--;
              bl--;
            }
            while(al >= 0){
                sum = (int)(a.charAt(al)-'0') + carries;
                res = String.valueOf(sum%2) + res;
                carries = sum/2;
                al--;
            }
            if(carries == 1){
                res = "1" + res;
            }
            //System.out.println("a>b的字符串："+res);
            return res;
        }else{
            while(al >= 0){
                sum =  (int)(a.charAt(al)-'0') + (int)(b.charAt(bl) - '0') + carries;
                res = String.valueOf(sum%2) + res;
                carries = sum/2;
                al--;
                bl--;
            }
            while(bl >= 0){
                sum = (int)(b.charAt(bl)-'0') + carries;
                res = String.valueOf(sum%2) + res;
                carries = sum/2;
                bl--;
            }
            if(carries == 1){
                res = "1" + res;
            }
            //System.out.println("a<b的字符串："+res);
            return res;
        }



    }
}