Merge pull request careermonk#7 from Amay22/master

Added function to merge multiple sorted LL using Priority Queue.

Author: careermonk

src/chapter03linkedlists/MergeKSortedLists.java

@@ -12,6 +12,43 @@
 
 package chapter03linkedlists;
 
+import java.util.ArrayList;
+import java.util.PriorityQueue;
+
 public class MergeKSortedLists {
 	//Refer Priority Queues
+    public static ListNode mergeKLists(ArrayList<ListNode> lists) {
+        if (lists.isEmpty()) {
+            return null;
+        }
+        //PriorityQueue is a sorted queue
+        PriorityQueue<ListNode> pq = new PriorityQueue<>(lists.size(), (ListNode a, ListNode b) -> {
+            if (a.getData() > b.getData) {
+                return 1;
+            } else if (a.getData() == b.getData()) {
+                return 0;
+            } else {
+                return -1;
+            }
+        });
+        //add first node of each list to the queue
+        for (ListNode list : lists) {
+            if (list != null) {
+                pq.add(list);
+            }
+        }
+        ListNode head = new ListNode(0);
+        ListNode p = head;
+        while (pq.size() > 0) {
+            //poll() retrieves and removes the head of the queue - q.
+            ListNode temp = pq.poll();
+            p.next = temp;
+            //keep adding next element of each list
+            if (temp.next != null) {
+                pq.add(temp.next);
+            }
+            p = p.next;
+        }
+        return head.next;
+    }
 }