[leetcode]67. Add Binary 二进制加法

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"

注意：

StringBuilder.insert(int offset,  char c) 表示将char放入offset这个偏移量

Character.forDigit(int digit, int radix) 返回radix进制下，该digit所代表的char

代码

 class Solution {
     public String addBinary(String a, String b) {
         StringBuilder sb = new StringBuilder(); // new a StringBuilder because it will be easy to append

         char[] arrayA = a.toCharArray();// convert char to array
         char[] arrayB = b.toCharArray();// convert char to array

         int i = a.length()-1;
         int j = b.length()-1;
         int carry = 0; 

         while(i>=0 || j>=0 || carry >0){
             int updateA = i>=0 ? arrayA[i--]-'0':0;
             int updateB = j>=0 ? arrayB[j--]-'0':0;
             int sum = updateA + updateB + carry;
             sb.insert(0, Character.forDigit(sum%2, 10)); // I convert this sum to binary then insert to sb
             carry = sum/2; // coz it is binary, carry would be sum/2
         }
         return sb.toString();  //  convert StringBuilder to String
     }
 }