67. Add Binary

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

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

Thought

Add binary with same concept in decimal, the different is carry when sum is reached to 2 instead 10. Use StringBuffer to keep the results, and insert the value at 0-index instead of appending to the last.

carry   1 1 1 1 1
    A     0 1 1 0 1
  + B     1 0 1 1 1
-----   -----------
A + B   1 0 0 1 0 0 = 36 (decimal)

Solution

public class Solution {
    public String addBinary(String a, String b) {
        StringBuilder res = new StringBuilder();
        int i = a.length() - 1, j = b.length() - 1, carry = 0;
        while (i >= 0 || j >= 0) {
            int sum = carry;
            // char -> ASCII value: '0' -> 48, '1' -> 49,
            // e.g., subtract '0' from '1' and we have 1
            if (i >= 0) sum += a.charAt(i--) - '0';
            if (j >= 0) sum += b.charAt(j--) - '0';
            carry = sum / 2;
            res.insert(0, String.valueOf(sum % 2));
        }

        if (carry == 1) res.insert(0, '1');
        return res.toString();
    }
}