MultiplyStrings

README.md

@@ -140,6 +140,7 @@ This repository contains the solutions to problems done from various resources f
 ## Linked List
 - [x] [Add Two Numbers - Leetcode](https://github.com/kalpak92/TechInterview2020/blob/master/src/Leetcode/AddTwoNumbers.java)
 - [x] [Add Two Numbers, where numbers are given from MSB to LSB - Leetcode](https://github.com/kalpak92/TechInterview2020/blob/master/src/Leetcode/AddTwoNumbersMSBToLSB.java)
+- [x] [Multiply Strings - Leetcode]()
 - [x] [Reverse a Linked List - Leetcode](https://github.com/kalpak92/TechInterview2020/blob/master/src/Leetcode/ReverseLinkedList.java)
 - [x] [Reverse a Linked List from Node m to Node n - Leetcode](https://github.com/kalpak92/TechInterview2020/blob/master/src/Leetcode/ReverseSublist.java)  
 - [x] [Reverse Nodes in k group - Leetcode](https://github.com/kalpak92/TechInterview2020/blob/master/src/Leetcode/ReverseLinkedListInKgroup.java)

src/Leetcode/MultiplyStrings.java

@@ -0,0 +1,70 @@
+package Leetcode;
+
+/**
+ * @author kalpak
+ *
+ * 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.
+ *
+ *
+ * Example 1:
+ * Input: num1 = "2", num2 = "3"
+ * Output: "6"
+ *
+ * Example 2:
+ * Input: num1 = "123", num2 = "456"
+ * Output: "56088"
+ *
+ *
+ * Constraints:
+ *
+ * 1 <= num1.length, num2.length <= 200
+ * num1 and num2 consist of digits only.
+ * Both num1 and num2 do not contain any leading zero, except the number 0 itself.
+ *
+ */
+
+public class MultiplyStrings {
+    public static String multiply(String num1, String num2) {
+        // Compute products from each pair of digits from num1 and num2
+        int n1 = num1.length();
+        int n2 = num2.length();
+
+        int[] products = new int[n1 + n2];
+
+        for (int i = n1 - 1; i >= 0; i--) {
+            for (int j = n2 - 1; j >= 0; j--) {
+                int d1 = num1.charAt(i) - '0';
+                int d2 = num2.charAt(j) - '0';
+
+                products[i + j + 1] += d1 * d2;
+            }
+        }
+
+        // carry each element over.
+        int carry = 0;
+
+        for (int i = products.length - 1; i >= 0; i--) {
+            int tmp = (products[i] + carry) % 10;
+            carry = (products[i] + carry) / 10;
+            products[i] = tmp;
+        }
+
+        // Output the result
+        StringBuilder result = new StringBuilder();
+        for(int i : products)
+            result.append(i);
+
+        while (result.length() != 0 && result.charAt(0) == '0')
+            result.deleteCharAt(0);
+
+        return result.length() == 0 ? "0" : result.toString();
+
+    }
+
+    public static void main(String[] args) {
+        System.out.println(multiply("123", "456"));
+    }
+}