Add Binary

https://leetcode.com/problems/add-binary/

Given two binary strings a and b, return their sum as a binary string.

Example 1:

Input: a = "11", b = "1"
Output: "100"

Example 2:

Input: a = "1010", b = "1011"
Output: "10101"

Solution:

That is a variation of a pretty popular problem. We should calculate a sum of 2 digits. .

Also here we are going to work with bits. Ok ok.

To convert char to int we can use this approach:

int a = '1' - '0';`
`

We will iterate over 2 strings at the same time. For it, we prepare 2 reader points.

Also, we will add the rest variable. Because 1 + 1 = 0 and rest = 1;

Then we just should check all cases.

NOTE. We are going to use StringBuilder. We cannot use Strings, because String in Java is immutable.

public String addBinary(String aa, String bb) {
    char[] aChars = aa.toCharArray();
    char[] bChars = bb.toCharArray();
     
    int rest = 0;
    int readerA = aChars.length - 1;
    int readerB = bChars.length - 1;
     
    StringBuilder sb = new StringBuilder();
     
    while (readerA >= 0 && readerB >= 0){
      int a = aChars[readerA] - '0';
      int b = bChars[readerB] - '0';
      int sum = a + b + rest;
      rest = sum/2;
      sb.append(sum % 2);
       
      readerA--;
      readerB--;
    }
     
    while(readerA >= 0){
      int a = aChars[readerA] - '0';
      int sum = a + rest;
      rest = sum/2;
      sb.append(sum % 2);
      readerA--;
    }
    while(readerB >= 0){
      int b = bChars[readerB] - '0';
      int sum = b + rest;
      rest = sum/2;
      sb.append(sum % 2);
      readerB--;
    }
    if (rest != 0){
      sb.append(rest);
    }
    return sb.reverse().toString();
  }