Added LinkedList Code in Java

Java/data_structures/linked_lists/Multiply_TwoNums_LL.java

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+/**
+ * MultiplyTwoNumbersLL
+ *
+ * Description:
+ * This class multiplies two numbers represented as singly linked lists.
+ * Each node contains a single digit, and digits are stored in forward order.
+ * The algorithm works by reversing the lists, multiplying digit by digit,
+ * shifting partial results according to place value, adding them together,
+ * and finally reversing the result to get the correct order.
+ *
+ * Use Cases:
+ * - Multiplying very large numbers that cannot fit into primitive data types
+ * - Working with numbers stored in linked list format in memory-efficient
+ * applications
+ *
+ * Time Complexity: O(m * n)
+ * - m = number of nodes in l1
+ * - n = number of nodes in l2
+ * - Each digit of l2 is multiplied with all digits of l1, and partial sums are
+ * added
+ *
+ * Space Complexity: O(m + n)
+ * - For storing the result linked list and intermediate partial products
+ */
+
+class ListNode {
+  int data;
+  ListNode next;
+
+  ListNode(int data) {
+    this.data = data;
+    this.next = null;
+  }
+}
+
+public class Multiply_TwoNums_LL {
+
+  /**
+   * Multiplies two numbers represented as linked lists
+   */
+  public ListNode multiplyTwoNumbers(ListNode l1, ListNode l2) {
+    if (l1 == null || l2 == null)
+      return new ListNode(0);
+
+    l1 = reverseList(l1); // Reverse to start from least significant digit
+    l2 = reverseList(l2);
+
+    ListNode result = new ListNode(0);
+    ListNode l2Ptr = l2;
+    int l2Pos = 0;
+
+    while (l2Ptr != null) {
+      // Multiply l1 with current digit of l2
+      ListNode partialProduct = multiplyWithSingleDigit(l1, l2Ptr.data);
+
+      // Add zeros to shift the partial product
+      partialProduct = addZeros(partialProduct, l2Pos);
+
+      // Add partial product to cumulative result
+      result = addTwoNumbers(result, partialProduct);
+
+      l2Ptr = l2Ptr.next;
+      l2Pos++;
+    }
+
+    return reverseList(result.next); // Reverse final result to correct order
+  }
+
+  private ListNode multiplyWithSingleDigit(ListNode l, int digit) {
+    ListNode dummy = new ListNode(0);
+    ListNode current = dummy;
+    int carry = 0;
+
+    while (l != null || carry != 0) {
+      int sum = carry + (l != null ? l.data * digit : 0);
+      carry = sum / 10;
+      current.next = new ListNode(sum % 10);
+      current = current.next;
+      if (l != null)
+        l = l.next;
+    }
+
+    return dummy.next;
+  }
+
+  private ListNode addZeros(ListNode l, int count) {
+    while (count-- > 0) {
+      ListNode newNode = new ListNode(0);
+      newNode.next = l;
+      l = newNode;
+    }
+    return l;
+  }
+
+  private ListNode addTwoNumbers(ListNode l1, ListNode l2) {
+    ListNode dummy = new ListNode(0);
+    ListNode current = dummy;
+    int carry = 0;
+
+    while (l1 != null || l2 != null || carry != 0) {
+      int sum = carry + (l1 != null ? l1.data : 0) + (l2 != null ? l2.data : 0);
+      carry = sum / 10;
+      current.next = new ListNode(sum % 10);
+      current = current.next;
+      if (l1 != null)
+        l1 = l1.next;
+      if (l2 != null)
+        l2 = l2.next;
+    }
+
+    return dummy.next;
+  }
+
+  private ListNode reverseList(ListNode head) {
+    ListNode prev = null, current = head;
+    while (current != null) {
+      ListNode next = current.next;
+      current.next = prev;
+      prev = current;
+      current = next;
+    }
+    return prev;
+  }
+
+  public static void printList(ListNode head) {
+    while (head != null) {
+      System.out.print(head.data);
+      head = head.next;
+    }
+    System.out.println();
+  }
+
+  public static void main(String[] args) {
+    // Test case 1: 342 * 465 = 159030
+    ListNode l1 = new ListNode(3);
+    l1.next = new ListNode(4);
+    l1.next.next = new ListNode(2);
+
+    ListNode l2 = new ListNode(4);
+    l2.next = new ListNode(6);
+    l2.next.next = new ListNode(5);
+
+    Multiply_TwoNums_LL mt = new Multiply_TwoNums_LL();
+    ListNode result = mt.multiplyTwoNumbers(l1, l2);
+
+    System.out.print("Product: ");
+    printList(result);
+  }
+}