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Computer Science Midterm 2 Cheat Sheet by

Some Helpful Big Oh Analysis

Big Oh
1+2+3+­4+....+ N
2^10 = 1024 ~~ 1000
O- notation is an upper bound so N is O(N) but it is also O(N^2)

Order of Growth Classi­fic­ations

Usually, nested for loops have a big O(N^2) because each of them runs n times. However, sometimes they can run less than n times.

for (int i =0; i<N; i++) ---> N times
for (int j =1; j<n; j - j*2)

Big O is n* log n times

Big Oh Complexity

Common Data Structure Operations

Arrays vs ArrayLists

They have a fixed size
Size can change
Much faster to add to
Adding to an arraylist is usually N
But when you reach the max, the computer doubles the limit every time you hit the limit so it takes O(N) times --> This is why it takes longer

Circular Linked Lists

Binary Search Tree

- Each node has a value
- Nodes with the values less than their parent are in the left
-Nodes with values greater than their parent are in the right subtree
- If equal, choose a side and stay consistent
- Insert from top of binary search tree and move down

Binary Tree Insertion

Appending Lists

BSTS to Lists

-Trees: are nodes with two pointers
-Doubly linked lists: also nodes with two pointers (allows for constant time access with one pointing to front and one pointing to back)

Complete Binary Tree

- Every non leaf node has two children
- All the leaves are at the same level
- There are 2N -1 or O(2N) nodes with N levels
- There are 2N-1 leaves with n levels

Priority Queues

- Minimum is first out
-Poll means remove the minimum each time
-List [0] will be smallest
-List [1] is smallest of all the ones that remain
-While a queue is first in first out, a priority queue is minium out first
-Shortest path


- Heap is an array-­based implem­ent­ation of a binary tree used for implem­enting priority queues and supports: insert, findMin, and deleteMin
-Using array minimizes storage (no explicit pointers) , faster too because children are locatd by index/­pos­ition in array
Deletion: remove root and replace with right most child and then bubble down filling left to right
- shape: tree filled at all levels (except perhaps last) and filled in left-t­o-right (complete binary tree
- value: each node has value smaller than both children
Min Heap:
- Minimal element is at root, index 1
-Maximal element has to be a leaf, because can't be greater than child
-Compl­exity of finding maximal elements, half nodes are trees --> O(n/2) so O(n)
- Second smallest element must be one level below root

Using An Array For a Heap

- Store node values in array starting at index 1
- For node with index k:
- left child: index 2*k
-right child: index 2*k +1
- parent: index k/2

Adding Values to Heap

- To maintain heap shape, must add new value in left-t­o-right order of last level
- This could violate heap property
- move value "­up" if too small
- Change places with parent if heap property is violated and stop when parent is smaller and stop when root is reached
-Pull parent down

Heap Add Implem­ent­ation


- Tries support add, contains, delete in O(w) time for words of length w
- Each node in a trie has a subtrie for every valid letter than can follow

Priority Queue Implem­ent­ations

Operat­ions: O(log n)
add - add element to last spot and bubble up
remove­/poll - remove root.min and take last element and bubble down

Graphs Vocabulary

- A collection of vertices and edges
- Edge connec­tions two vertices
- Direction can be imported, directed edge, directed graph
- Edge may have associated weight­/cost
- A vertex sequence is a path where vk and vk+1 are connected by an edge
- If some vertex is repeated, the path is a cycle
- A graph is connected if there is a path between any pair of vertices
-Artic­ulation Point breaks graph in two

Graphs DFS

Envision each vertex as a room
Go into a room, mark the room, choose an unused door, exit
Don't go into room you've already been in--> explore every vertex one time
qu is where we're going, visited is where we've been

Adjacency Lists and Matrix

Adjacency List: V+E spaces
Adjacency Matrix: V*E


Creating Adjacency Matrix

public int howLong(String [] connects, String [] costs){
    int [] [] adjMatrix = new int [connects.length][connects.length]' 
   for (int i =0; i<connects.length; i++){
      String [] edges - connects[i[.split(" "); 
      String [] weights = costs[i].split(" ");  
           for (int j =0; j<edges.length; j++){
                 adjMatrix[i][Integer.partseInt(edges[j])) = Integer.parseInt(weights[j]);

Analysis: Empirical vs. Mathem­atical

Empirical Analysis
Mathem­atical Analysis
Measure running times, plot, and fit curve
Analyze algorithm to estimate # ops as a function of input size
Easy to perform experi­ments
May require advanced mathem­atics
Model useful for predic­ting, but not for explaining
Model useful for predicting and explaining
Mathem­atical analysis is indepe­ndent of a particular machine or compiler; applies to machines not yet built.



- Can't always access comparable method (imple­ments .compareTo and uses Collec­tio­ns.sort and Arrays.sort)
-Sometimes must implement compar­ators in which you pass two objects
- Must implement .compare(T a, T b)
- Return <0 when a<b
- Return ==0 when a ==b
Return >0 when a>b

Comparator Example

Comparator Example

Linked List vs. ArrayList

Linked List
Separate elements in memory that all have pointers to each other
A collection of elements in order in memory
Collection of elements
Add, remove, for loops, sort themse­lves, clear
Remove First Element:
N Time because you don't have to shift something when it is in the front of the list
N^2 Time because everything stores sequen­tially so when you take something out you have to shift everything by 1
Remove Middle Index
Has a higher coeffi­cient and thus is slower: To get there takes time but to remove it is instan­taneous :O(N)
Faster: To get to middle element is instan­tan­enous but to remove it you have to shift it: O(N)
Best for adding­/re­moving front
Best for adding­/re­moving something from back/m­iddle


If you have N nodes, height is asking how many times you can divide by 2 --> expressed as the log base 2 of n
Good search tree is height is log n
Bad search tree is n
Balanced if left and right subtrees are height balanced and left and right heights differ by at most 1


-BruteA­uto­com­plete: stores data as a Term array and finds the top k matches by iterating through the array and pushes all terms starting with the prefix into a max priority queue sorted by weight and returns the top k terms from that priority queue
- not compared by weight and o organi­zation
-topMa­tches: O(n+mlogm)
- topMatch: O(n)
-Improving BruteA­uto­com­plete:
- had to iterate through every single term in the array because it did not know where the terms starting with the prefix were located aka array was unsorted.
- If we sort the array lexico­gra­phi­cally, then all the terms with the same prefix will be adjacent (Sorting takes O(n log n)
- Term: encaps­ulates a word/term and its corres­ponding weight
-Binary­Sea­rch­Aut­oco­mplete: finds Terms with a given prefix by performing a binary search on a sorted array of Terms
-TrieAu­toc­omp­lete: finds Terms with a given prefix by building a trie to store the Terms. To be efficient should only look at words whos maxsubtree weight is greater than the minimum

Autoco­mplete: Term class

- The term class encaps­ulates a comparable word weight pair
- Weight­Order: sorts in ascending weight order
-Revers­eWe­igh­tOrder: sorts in descending weight order
-Prefix­Order: which sorts by the first r characters
-If one or both words are shorter than r, we just use normal lexico­gra­phical sorting
- compare method must take O(r)

Autoco­mplete: Binary Search

- Find all the range of all the terms comparator considers equal to key
- Quickly return the first and last index respec­tively of an element in the input array which the comparator considers to be equal to key
- We specify the first and last index because there could be multiple Terms in which the comparator consider to be equal to key
- Collec­tio­ns.b­inary search does not guarantee first index of terms that match key, it gives an index

Autoco­mplete: Tries

- To completely eliminate terms which don't start with prefix, store in trie
- Navigate to the node repres­enting the string. The trie rooted at this node only contains nodes starting with this trie
- No matter how many words are in our trie, navigating this node takes the same amount of time

Autoco­mplete: Big-Oh

O(mlogm + n)
O(log(n) + m)
O(log(n) + (m + k)log(k))
n: number of terms in total
m: number of terms that match the prefix
k: desired number of terms
w: number of letters in the word

Common Recurr­ences and Their Solutions

Huffman: Compre­ssing

Huffman: Decomp­ressing

Graphs BFS

Visit everything that is one away, then everything that is two away...
Used to find shortest distance
takes a lot of space --> B^d

Bacon Number

Good Center - Has the most people closest to them
Chooses the best path, lowest number of edges
Actor Actor Repres­ent­ation//Vert­ices: actors or actres­ses­//E­dges: Two actors are adjacent (joined by a graph edge) if and only if they appear in the same movie
Movie Movie Repres­ent­ation// Vertices: Movies­//E­dges: Two movies are adjacent if they share a cast member
Actor Movie Repres­ent­ation//Vert­ices: Actors, actresses, and movies// Edges: An actor is connected to a movie if he or she appeared in that movie
1. Most vertices: Actor to movie
a. All of the vertices you had in actor to actor and all vertices in movie
to movie
2. Most edges: Actor to Actor

Erdos Number Part 2







Efficient sorting algorithms are usually recursive
Base Case: does not make a recursive call
For Linked Lists: Base case is always empty list or singular node/R­ecu­rsive calls make progress toward base case ( as argument)

Percol­ation Overview

- System percolates if top and bottom are connected by open sites
- Given a NxN grid, where each is site is open with probab­ility p*, what is the probab­ility that the system percol­ates?
- if p>p*, system most likely percolates
- if p< p*, system does not percolate
-All simula­tions, whether using Percol­ati­onDFS, Percol­ati­onD­FSFast, or Percol­ation UF with any implem­ent­ation of union-find will be at least O(N2)
- Finding the threshold
- Initialize NxN grid of sites as blocked
- Randomly open sites until system percolates
- Percentage of pen sites gives an approx­imation of p*
How do you get random cells to open and not open same shell more than once:
- Make points out of the cells
- Shuffle them gets a random ordering of all the point where each one
occurs once time
- Go through and repeatedly open each

Percol­ation Solution 1: Depth First Search

- Try searching from all of the open spots on the top row
- Search from all legal adjacent spots you have not visited
- Recurse until you can't search any further or have reached the bottom row
- Try all the legal adjacent spots (what makes it recursive is that we do the
same problem but at a different place)
- Base Cases:
- Out of bounds
- Blocked
- Already full/v­isited
Percol­ati­onDFS sets each grid cell to OPEN and runs a DFS from each open cell in the top (index­-zero) row to mark the cells reachable from them as FULL. In the new model Percol­ati­onD­FSFast, you'll make this implem­ent­ation more efficient by only checking the cell being opened to see if it results in more FULL cells, rather than checking every cell reachable from the top row.
-Perco­lation DFS and DFSFast run in O(N) because it iterates through only the bottom row to check if it is full

Percol­ation DFSFast

- Why is this an Improv­ement: An improv­ement because we don't have to search from the top:
- Don't have to start from the top and go down
- For the cells that are adjacent, now search from that spot
- If one of my neighbors is full, I am full
-Don't have to redfs things you've already seen
Percol­ation DFS Fast
1. Create a grid
2. Set them all to blocked
3. Protected void update­OnOpen
4. Clear everything from being full
5. Dfs checks base cases
a. If not in bounds, return
b. If cell is full or not open, return
Otherwise try all neighbors recurs­ively

Percol­ation Solution 2: Union- Find

- Create an object for each site (each cell) (Vtop as N*N, Vbottom as N^2 +1)
- Percolates if vtop is connected to vbottom
- One call that you have to make --> union find
- For every cell, give it an index
- Becomes proble­matic when n is too long
- Look at ultimate parent making path short to find parent at constant time
- Run time is O(1) because we simply check if vtop and vbottom are in the same set
-IUnio­nFI­nd.find is called from both connected and union to find sets that p and q belong to

Percol­ation Method Score Board

Tree Traversals InOrder

Visit left sub-tree, process root, visit right subtree
Increasing order
- Follow path and In order is when you do outline and you hit it the second time

Tree Traversals PreOrder

Process root, then visit left subtree, then visit right subtree
Good for readin­g/w­riting trees
- When you follow the outline and preorder is just when you hit it for the first time

Tree Traver­sals: PostOrder

Visit left subtree, right subtree, process root
Good for deleting trees
When you follow the outline and postorder is when you hit it going up

Recursion with ListNodes in return statement

public ListNode<Integer> convertRec (ListNode<String> list):
if (list == null) return null; 
return new ListNode<Integer> (, convertRev(;

Doubly Linked Lists

List Node first = new ListNode <"cherry", null, null>; 
List Node fig = new ListNode <"fig", first, null>;
List Node mango = new ListNode <"mango", fig, null>; 
first.right = fig; 
fig.right = mango;

Data Compre­ssion

Lossless: Can recover exact data
Lossy: Can recover approx­imate data
- Use when you don't care, photos can't tell the differ­ence, can compress it


Huffman: Text Compre­ssion

In the trie, 0 is left, 1 is right
Make the ones that occur most often the shortest path
Ones that rarely occur can be long
Ones that never occur can be as long as we want
Look at it 8 bits at a time
Building: Combine minimally weighted trees --> Greedy
Bad Huffman Tree: when different character occurs once
Good Hufman Tree: One character occurs multiple times
Alphabet size and run time and compre­ssion rate:
- Alphabet size has a big impact on run time because alph size tell syou how big the tree will be
- The number of leaves is equal to the size of your allphabet, so you have 2^k nodes in your tree
- Amount of compre­ssion is frequency that it occurs
○ 256 characters that occur the same amount of time is bad compre­ssion
○ Huffman takes advantage of the fact that some characters occur more often than others

Creating Huffman Tree

Huffman: TreeTi­ghten APT

Big Oh for Huffman Encodings

Read in tree data: O(k)
Decode bit string with tree: O(n)

Actor Actor Graph

Movie Movie Graph

Actor Movie Graph

DFS Recursion

Erdos Number Part 1

Creating Adjacency Matrix

public int howLong(String [] connects, String [] costs){
    int [] [] adjMatrix = new int [connects.length][connects.length]' 
   for (int i =0; i<connects.length; i++){
      String [] edges - connects[i[.split(" "); 
      String [] weights = costs[i].split(" ");  
           for (int j =0; j<edges.length; j++){
                 adjMatrix[i][Integer.partseInt(edges[j])) = Integer.parseInt(weights[j]);


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