SDE - 1

So this problem can be solved using Heap. 
I have created max Heap of pair in C++. The first pair will be the element and the second will be the index of that element in the given array.
Then I used the sliding window technique.Firstly I have inserted the elements in the heap till their size is < k. As soon as its size becomes >= k then I am taking out its top.first element (Max element in case of the max heap) and insert it to output vector and then pop out the elements which are of no use that is not there in the current slide.
This way I did this problem.

Here I have used a slow and fast pointer approach.
The idea is to have 2 pointers: slow and fast. Slow pointer takes a single jump and corresponding to every jump slow pointer takes, fast pointer takes 2 jumps. If there exists a cycle, both slow and fast pointers will reach the exact same node. If there is no cycle in the given linked list, then fast pointer will reach the end of the linked list well before the slow pointer reaches the end or null.
Initialize slow and fast at the beginning.
Start moving slow to every next node and moving fast 2 jumps, while making sure that fast and its next is not null.
If after adjusting slow and fast, if they are referring to the same node, there is a cycle otherwise repeat the process
If fast reaches the end or null then the execution stops and we can conclude that no cycle exists.

In this question, we have to find the number of islands. So I just simply applied the DFS from any 1 in the 2D matrix and mark visited in the visited matrix and keep the counter on each DFS call.

Tip 1 : Read Galvin for OS thoroughly