Priority Queue : In Java Priority Queue is a queue which keeps its elements sorted as per their natural order( example in ascending orders for Integer, alphabetical a-z for alphabets) or using a custom Comparator at the time of creation you can custom sort it. It has most method similar to a queue add, clear, poll, peek As priority queue have to compare elements to keep in order so elements must be comparable otherwise, it will throw ClassCastException . As null can not be compared so you are not allowed to insert null too A program to sort priority queue using Lambda function in Java c lass PriorityQueueLambdaJava { public List<Integer> KFrequent( int [] nums, int k) { Map<Integer, Integer> map = new HashMap<>(); for ( int j: nums) { map .put(j, map .getOrDefault(j, 0 ) + 1 ); } PriorityQueue<Map.Entry<Integ...