Multiply Strings editorial added

Regular-Question-Answers/multiply-strings.cpp

@@ -0,0 +1,76 @@
+//Author: Rampia Perente
+// Question Link: https://leetcode.com/problems/multiply-strings
+
+/* 
+Problem Statement: Given two non-negative integers 
+num1 and num2 represented as strings, 
+return the product of num1 and num2, 
+also represented as a string.
+
+Note: 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) {
+
+        /*Handle the edge case: if either number is zero, 
+        it will lead the result to zero
+        */
+        if(num1 == "0" || num2 == "0")
+        return "0";
+
+        /*
+        Initialize the vector that will hold the result, 
+        the size of multiplication cannot exceed 
+        the sum of sizes of two numbers
+        */
+        vector<int> result(num1.size() + num2.size(), 0);
+
+        for(int i = num1.size() - 1; i >= 0; i--)
+        for(int j = num2.size() - 1; j >= 0; j--)
+        {
+            /*
+            Basic multiplication step: Since we are operating on strings, 
+            we have to convert the string to integer. 
+            '0' has an ASCII value of 48, '1' -> 49, ...., '9' -> 57, 
+            so num[] - '0' will give the integer value 
+            of each character, for example, 57 - 48 = 9
+            */
+            int multiply = (num1[i] - '0') * (num2[j] - '0');
+            /*
+            Add the carry and the multiplication result 
+            at the current position of the result vector
+            */
+            int sum = multiply + result[i + j + 1];
+            /*
+            If the sum is greater than 9, take the rightmost 
+            digit of it and store it in current position
+            */
+            result[i + j + 1] = sum % 10;
+            /*
+            Add the carry that occured by dividing 
+            the sum by 10 to the next position in result 
+            vector(we are going right to left)
+            */
+            result[i + j] += sum / 10;
+        }
+
+        string answer;
+
+        //Skip leading zeros
+        int i = 0;
+        while(result[i] == 0)
+        i++;
+
+        /*
+        Convert each digit of integer number to string
+        and append it to the string answer
+        */
+        while(i < result.size())
+        answer += to_string(result[i++]);
+
+        return answer;
+    }
+};