43. Multiply Strings (大数乘法)

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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[] pos = new int[m + n];

     for(int i = m - 1; i >= 0; i--) {
         for(int j = n - 1; j >= 0; j--) {
             int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
             int p1 = i + j, p2 = i + j + 1;
             int sum = mul + pos[p2];

             pos[p1] += sum / 10;
             pos[p2] = (sum) % 10;
         }
     }  

     StringBuilder sb = new StringBuilder();
     for(int p : pos)
         if(!(sb.length() == 0 && p == 0))
             sb.append(p);
     return sb.length() == 0 ? "0" : sb.toString();

     }
 }