Add RandomNode in lists section (#2851)

Co-authored-by: Yang Libin <contact@yanglibin.info>

src/main/java/com/thealgorithms/datastructures/lists/README.md

@@ -27,4 +27,5 @@ The `next` variable points to the next node in the data structure and value stor
 3. `CountSinglyLinkedListRecursion.java`: Recursively counts the size of a list.
 4. `CreateAndDetectLoop.java` : Create and detect a loop in a linked list.
 5. `DoublyLinkedList.java` : A modification of singly linked list which has a `prev` pointer to point to the previous node.
-6. `Merge_K_SortedLinkedlist.java` : Merges K sorted linked list with mergesort (mergesort is also the most efficient sorting algorithm for linked list).
+6. `Merge_K_SortedLinkedlist.java` : Merges K sorted linked list with mergesort (mergesort is also the most efficient sorting algorithm for linked list).
+7. `RandomNode.java` : Selects a random node from given linked list and diplays it.

src/main/java/com/thealgorithms/datastructures/lists/RandomNode.java

@@ -0,0 +1,93 @@
+/** Author : Suraj Kumar
+ * Github : https://github.com/skmodi649
+ */
+
+/** PROBLEM DESCRIPTION :
+ * There is a single linked list and we are supposed to find a random node in the given linked list
+ */
+
+/** ALGORITHM :
+ * Step 1 : START
+ * Step 2 : Create an arraylist of type integer
+ * Step 3 : Declare an integer type variable for size and linked list type for head
+ * Step 4 : We will use two methods, one for traversing through the linked list using while loop and also increase the size by 1
+ *
+ * (a) RandomNode(head)
+ * (b) run a while loop till null;
+ * (c) add the value to arraylist;
+ * (d) increase the size;
+ *
+ * Step 5 : Now use another method for getting random values using Math.random() and return the value present in arraylist for the calculated index
+ * Step 6 : Now in main() method we will simply insert nodes in the linked list and then call the appropriate method and then print the random node generated
+ * Step 7 : STOP
+ */
+
+
+package com.thealgorithms.datastructures.lists;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+public class RandomNode {
+    private List<Integer> list;
+    private int size;
+    private static Random rand = new Random();
+
+    static class ListNode {
+        int val;
+        ListNode next;
+
+        ListNode(int val) {
+            this.val = val;
+        }
+    }
+
+    public RandomNode(ListNode head) {
+        list = new ArrayList<>();
+        ListNode temp = head;
+
+        // Now using while loop to traverse through the linked list and
+        // go on adding values and increasing the size value by 1
+        while (temp != null) {
+            list.add(temp.val);
+            temp = temp.next;
+            size++;
+        }
+    }
+
+    public int getRandom() {
+        int index = rand.nextInt(size);
+        return list.get(index);
+    }
+
+    // Driver program to test above functions
+    public static void main(String[] args) {
+        ListNode head = new ListNode(15);
+        head.next = new ListNode(25);
+        head.next.next = new ListNode(4);
+        head.next.next.next = new ListNode(1);
+        head.next.next.next.next = new ListNode(78);
+        head.next.next.next.next.next = new ListNode(63);
+        RandomNode list = new RandomNode(head);
+        int randomNum = list.getRandom();
+        System.out.println("Random Node : " + randomNum);
+    }
+}
+
+
+/**
+ * OUTPUT :
+ * First output :
+ * Random Node : 25
+ * Second output :
+ * Random Node : 78
+ * Time Complexity : O(n)
+ * Auxiliary Space Complexity : O(1)
+ * Time Complexity : O(n)
+ * Auxiliary Space Complexity : O(1)
+ */
+
+/** Time Complexity : O(n)
+ * Auxiliary Space Complexity : O(1)
+ */