[LeetCode] Add Binary

Problem

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"

Solution

//using StringBuilder.insert(0, "content") is silly
//just reverse and convert to string

class Solution {
    public String addBinary(String a, String b) {
        int i = a.length()-1, j = b.length()-1, carry = 0;
        StringBuilder sb = new StringBuilder();
        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();
    }
}