43. Multiply Strings

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"

Note:

1. The length of both num1 and num2 is < 110.

2. Both num1 and num2 contain only digits 0-9.

3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.

4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

class Solution {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] p = new int[m+n];
        
        
        for(int i = m -1 ; i >= 0; i--){
            for(int j = n - 1; j >= 0; j--){
                int tmp = (num1.charAt(i) - '0')*(num2.charAt(j) - '0');
                
                int p1 = i+j;
                int p2 = i+j+1;
                int sum = tmp + p[p2];
                p[p1] += sum/10;
                p[p2] = sum % 10;
            }
        }
        
        StringBuilder sb = new StringBuilder();
        
        for(int i : p ){
            if(!(sb.length() == 0 && i == 0)) sb.append(i);
        }
        
        return sb.length() == 0 ? "0" : sb.toString();
    }
}