Add Binary Strings

Add Binary Strings
Difficulty: MediumAccuracy: 23.25%Submissions: 79K+Points: 4
Given two binary strings s1 and s2 consisting of only 0s and 1s. Find the resultant string after adding the two Binary Strings.
Note: The input strings may contain leading zeros but the output string should not have any leading zeros.

Input: s1 = "1101", s2 = "111"
Output: 10100
Explanation:
 1101
+ 111
10100
Input: s1 = "00100", s2 = "010"
Output: 110
Explanation: 
  100
+  10
  110
Constraints:
1 ≤s1.size(), s2.size()≤ 10

class Solution {
    public String addBinary(String s1, String s2) {
        int i = s1.length() - 1, j = s2.length() - 1;
        if (i == -1) return s2;
        if (j == -1) return s1;

        StringBuilder res = new StringBuilder();
        char ch1, ch2, carry = '0';
        int temp;

        while (i >= 0 || j >= 0) {
            if (i >= 0)
                ch1 = s1.charAt(i);
            else
                ch1 = '0';

            if (j >= 0)
                ch2 = s2.charAt(j);
            else
                ch2 = '0';

            temp = (ch1 - '0') + (ch2 - '0') + (carry - '0');
            if (temp == 0) {
                res.append('0');
                carry = '0';
            } else if (temp == 1) {
                res.append('1');
                carry = '0';
            } else if (temp == 2) {
                res.append('0');
                carry = '1';
            } else {
                res.append('1');
                carry = '1';
            }

            i--;
            j--;
        }
        if (carry == '1') res.append(carry);
        int len = res.length();
        while (len > 0 && res.charAt(len - 1) == '0') len--;
        if (len == 0) return "0";
        return res.reverse().toString();
    }
}