Leetcode Multiply Strings

public String multiply(String num1, String num2) {
        String n1 = new StringBuilder(num1).reverse().toString();
        String n2 = new StringBuilder(num2).reverse().toString();
        int[] d = new int[num1.length() + num2.length()];    
        
        //multiply each digit and sum at the corresponding positions
        for(int i=0; i < n1.length(); i++){
            for(int j=0; j < n2.length(); j++){
                d[i+j] += (n1.charAt(i) - ‘0‘) *(n2.charAt(j) - ‘0‘);
            }
        }
        
        StringBuilder sb = new StringBuilder();
        
        //calculate each digit
        for(int i=0; i< d.length; i++){
            int mod = d[i]%10;
            int carry = d[i]/10;
            if(i+1 < d.length){
                d[i+1] += carry;
            }
            sb.insert(0, mod);
        }
        
        //remove front 0‘s
        while(sb.charAt(0) == ‘0‘ && sb.length() >1) {
            sb.deleteCharAt(0);
        }
        return sb.toString();
    }