# Lab 03

Feedback form (Please fill this form, to give feedback about this lab)

Here is a pdf version of the ppt I covered during the lab. And the codes that I (skimmed through) / (demonstrated live).

content.pdf (The pdf version of the ppt shown)

STLList.cpp (The STL List implementation) *(FIX- Please check the new print function earlier one was inefficient. In the earlier one I by mistake passed the list by value and then used another list just to pop out the stuff. A better approach is to pass the list by reference and iterate through all the elements using iterators.)*

STLStack.cpp (The STL Stack implementation)

STLQueue.cpp (The STL Queue implementation)

STLDeque.cpp (The STL Deque implementation) *(FIX - The deque is NOT implemented using DLL (Doubly linked list) internally, it is actually implemented using the concept of dynamic arrays. This is because it supports accessing any random element in O(1), i.e DQ[x] can be accessed for any x in O(1). The broad idea of dynamic array technique is that you have a structure like SLL / DLL, but each node of SLL/DLL does not store an element instead it stores an array. The in-depth details are out of scope. This fix is primarily for Group 04.)*

SLLStack.cpp (Our own implementation of a stack using SLL (Singly Linked List)) **(Can be useful for PS2 - D)** *(FIX - The function printList cannot be accessed by the class MyStack because the inheritance done is private. When we say class A inherits from class B, in a way as class A : B { // Code } it is equivalent to saying class A : private B { // Code }. In simpler terms, if we drop the word “public”/”private” then by default for classes it is meant to be “private”, whereas for “struct” by default it is meant to “public”. Since we are using classes, so all the inherited functions are privately inherited, so we cannot access the inherited functions through the main(), i.e because main() is out of the scope of MyStack. This fix is primarily for Group 04)*

## Functions we used -

advance (Used for moving an iterator in positive / negative direction from its current position)

## Challenge Questions -

Q) PS1 Variant - Instead of finding the median, i.e the N/2 th element, can you do the same question for the N/3 rd element. (assume integer division for cases when N is not divisible by 3). So now I am asking you to add all the N/3 rd elements in sorted order and print the sum. (**Note** - Solve this in O(N*logN) )

## Solution

Just try to maintain a 1:2 ratio in the 2 priority queues instead of a 1:1 ratio.(Assuming you know the priority queue solution, if not then please watch this for the first 3 minutes, to get a clear understanding)

Q) PS1 Final version - Solve PS1 C but with the additional constraint that all integers in array A[] are non-negative integers which are not more than 100000, in just O(N). O(N) means you cannot use priority_queue any more. In simpler terms I have simplfied the constraints since now the value of elements in A[] can be atmost 100000 but at the same time I want you to solve this question in O(N) not O(N*logN)(**Note** - In this question stick with the original definition of median, i.e N/2 th element. This question is independent of the question mentioned above)

**Hint 1 - Use counting sort + something ??**

**Hint 2 - Try to solve the question in reverse order, meaning first find the median for all the n elements, then for the first n - 1 elements, then for the first n - 2 elements and so on. (The solution that I have only works in reverse order) :)**

## Solution

My solution.cppPS - This solution does not do COUNTING SORT (since I implemented it for PS1 C, where I could not use counting sort since the numbers could be large but apart from the counting sort the remaining logic is O(N))

What is the broad idea ?

What I do is first keep the entire array in sorted order. Now I would rephrase the question to iteratively finding the median and removing an element (i.e the latest element) from this array.

Example -

- For simplicity in this example, when N = even, we will just say that the median is the leftward element out of the 2 in between
- N = 6, array = [1, 3, 2, 5, 7, 4]
- sorted_array = [1, 2, 3, 4, 5, 7]
- My algo in outline -
- Find median, i.e 3
- Now remove 4 (because in original array it was the last element)
- So sorted_array' = [1, 2, 3, x, 5, 7] (Here x denotes a deleted element)
- Again find median for this array, i.e = 3
- Now remove 7 (second last element in original array)
- So sorted_array'' = [1, 2, 3, x, 5, x]
- Now median is = 2
- Now remove 5
- So array is [1, 2, 3, x, x, x]
- Median becomes 2.
- Now remove 2
- Array becomes [1, x, 3, x, x, x]
- Median is 1
- Array becomes [1, x, x, x, x, x]
- Median is 1.
- Sum up all the medians.

- Now let us define some notation, a "x" denotes an "inactive cell" and a "number" denotes an "active cell".
- Also imaginve a pointer to the median of the sorted_arrays[], i.e initally it is at 3, then stays at 3. Then moves to 2, then statys at 2, the moves to 1 and then stays at 1.
- So if you carefully observe, in the above given method, we are ensured that after the deletion of an element in the sorted array at any given step, the median either remains at the same position, or moves to the next/previous "active cell".
- Do note, that actually moving slowly to the next/previous "active cell" can be slow and result in O(N^2) algorithm, so we need a fast method to quickly jump to the next/previous active cell
- For this I keep to arrays left_of[] and right_of[], here left_of[a] should denote the immediate "active cell" to the left of the indice "a". Initially left_of[a] = a, for all a. Similarly let right_of[a] denote the the immediate "active cell" to the right of the indice "a". Initially right_of[a] = a, for all a.
- Now, we will basically constantly maintain these 2 arrays for all the elements, in real time. So whenever we need to move to next / previous "active cell" we would simply look up in this array. The details of this portion are a bit hard to explain so I would recommend you to either go through the code / meet me after a lab to discuss this

Q) NA