LeetCode 43. Multiply Strings

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LeetCode 43. Multiply Strings

43. Multiply Strings

Difficulty: Medium

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:

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

Example 2:

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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.

Solution

Language: Java

乘法的过程
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num1[i] * num2[j] 的结果将会落在 [i + j, i + j + 1] 

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class Solution {
      public String multiply(String num1, String num2) {
            if (num1 == null || num2 == null || num1.length() == 0 || num2.length() == 0
                || num1.equals("0") || num2.equals("0")) {
                  return "0";
           }
            
            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();
     }
}