Two ways to get numeric value in string

https://discuss.leetcode.com/topic/5172/simple-accepted-java-solution

Character.getNumericValue(char)

public class Solution {
    public String addBinary(String a, String b) {
        if(a == null || a.isEmpty()) {
            return b;
        }
        if(b == null || b.isEmpty()) {
            return a;
        }
        char[] aArray = a.toCharArray();
        char[] bArray = b.toCharArray();
        StringBuilder stb = new StringBuilder();

        int i = aArray.length - 1;
        int j = bArray.length - 1;
        int aByte;
        int bByte;
        int carry = 0;
        int result;

        while(i > -1 || j > -1 || carry == 1) {
            aByte = (i > -1) ? Character.getNumericValue(aArray[i--]) : 0;
            bByte = (j > -1) ? Character.getNumericValue(bArray[j--]) : 0;
            result = aByte ^ bByte ^ carry;
            carry = ((aByte + bByte + carry) >= 2) ? 1 : 0;
            stb.append(result);
        }
        return stb.reverse().toString();
    }
}

https://leetcode.com/submissions/detail/95634744/

string.charAt(i) - '0'

public class Solution {
    public String addBinary(String a, String b) {
        int i = a.length() - 1, j = b.length() - 1;
        int carry = 0;
        StringBuilder res = new StringBuilder();
        while(i >= 0 || j >= 0 || carry != 0){
            int sum = carry;
            if(i >= 0){
                sum += a.charAt(i) - '0';    
            }

            if(j >= 0){
                sum += b.charAt(j) - '0';
            }

            res.append(sum % 2);
            carry = sum / 2;
            i--;
            j--;
        }
        return res.reverse().toString();
    }
}