43. Multiply Strings.java

class Solution {
    public String multiply(String num1, String num2) {
        if (num1.equals("0") || num2.equals("0")) return "0";  
        
        
        int[] result = new int[num1.length() + num2.length()];
        
        
        for (int i = num1.length() - 1; i >= 0; i--) {
            for (int j = num2.length() - 1; j >= 0; j--) {
                
                int digit1 = num1.charAt(i) - '0';
                int digit2 = num2.charAt(j) - '0';
                int mul = digit1 * digit2;
                
                
                int posLow = i + j + 1;
                int posHigh = i + j;
                
               
                int sum = mul + result[posLow];
                
                result[posLow] = sum % 10; 
                result[posHigh] += sum / 10; 
            }
        }
        
        
        StringBuilder product = new StringBuilder();
        for (int num : result) {
            
            if (!(product.length() == 0 && num == 0)) {
                product.append(num);
            }
        }
        
       
        return product.length() == 0 ? "0" : product.toString();
    }
}