Add Binary.java

/*
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"

time = O(max(m,n))
space = O(1)
*/

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder();
        int i = a.length() - 1, j = b.length() - 1, carry = 0;
        while (i >= 0 || j >= 0) {
            int sum = carry;
            if (i >= 0) {
                sum += a.charAt(i--) - '0';
            }
            if (j >= 0) {
                sum += b.charAt(j--) - '0';
            }
            sb.append(sum % 2);
            carry = sum / 2;
        }
        if (carry != 0) {
            sb.append(carry);
        }
        return sb.reverse().toString();
    }
}