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

For example,

a = "11"

b = "1"

Return "100".

 

tag： 字符asc码 。字符和数字加减法 。 '1' - '0' = 1 .  '1' - 0 != 0

 

public class Solution {    public String addBinary(String a, String b) {        StringBuilder result = new StringBuilder();        if(a == null && b == null) {            return result.toString();        }        if(a == null) {            return b;        }        if(b == null) {            return a;        }        int aIndex = a.length() - 1;        int bIndex = b.length() - 1;        int carry = 0;                while(aIndex >= 0 && bIndex >= 0) {            int sum = (int)((a.charAt(aIndex) - '0') + (b.charAt(bIndex) - '0') + carry ) % 2;            carry = (int)((a.charAt(aIndex) - '0') + (b.charAt(bIndex) - '0') + carry ) / 2;            result.append(sum);            aIndex--;            bIndex--;        }                while(aIndex >= 0){            int sum = (int)((a.charAt(aIndex) - '0') + carry ) % 2;            carry = (int)((a.charAt(aIndex) - '0') + carry) / 2;            result.append(sum);            aIndex--;        }                 while(bIndex >= 0){            int sum = (int)((b.charAt(bIndex) - '0') + carry ) % 2;            carry = (int)((b.charAt(bIndex) - '0') + carry) / 2;            result.append(sum);            bIndex--;        }                if(carry == 1) {            result.append("1");        }                 return result.reverse().toString();                    }}