Update multiply-strings.cpp

Author: kamyu104

C++/multiply-strings.cpp

@@ -2,6 +2,27 @@
 // Space: O(m + n)
 
 class Solution {
+public:
+    string multiply(string num1, string num2) {
+        string result(num1.size() + num2.size(), '0');
+        for (int i = num1.size() - 1; i >= 0; --i) {
+            for (int j = num2.size() - 1; j >= 0; --j) {
+                int sum = (num1[i] - '0') * (num2[j] - '0') + (result[i + j + 1] - '0');
+                result[i + j + 1] = sum % 10 + '0';
+                result[i + j] += sum / 10;
+            }
+        }
+        int pos = result.find_first_not_of('0');
+        if (pos != string::npos) {
+            return result.substr(pos);
+        }
+        return "0";
+    }
+};
+
+// Time:  O(m * n)
+// Space: O(m + n)
+class Solution2 {
 public:
     string multiply(string num1, string num2) {
         const auto char_to_int = [](const char c) { return c - '0'; };
@@ -32,7 +53,7 @@ class Solution {
 // Time:  O(m * n)
 // Space: O(m + n)
 // Define a new BigInt class solution. 
-class Solution2 {
+class Solution3 {
 public:
     string multiply(string num1, string num2) {
         return BigInt(num1) * BigInt(num2);