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.

 1 class Solution {
 2     public String multiply(String num1, String num2) {
 3     int m = num1.length(), n = num2.length();
 4     int[] pos = new int[m + n];
 5    
 6     for(int i = m - 1; i >= 0; i--) {
 7         for(int j = n - 1; j >= 0; j--) {
 8             int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0'); 
 9             int p1 = i + j, p2 = i + j + 1;
10             int sum = mul + pos[p2];
11 
12             pos[p1] += sum / 10;
13             pos[p2] = (sum) % 10;
14         }
15     }  
16     
17     StringBuilder sb = new StringBuilder();
18     for(int p : pos) 
19         if(!(sb.length() == 0 && p == 0)) 
20             sb.append(p);
21     return sb.length() == 0 ? "0" : sb.toString();
22 
23     }
24 }