X_400614 Data Structures and Algorithms
Index
Chapter 1 - The Role of Algorithms in Computing...........................................................................................1
Chapter 2 - Getting Started ...............................................................................................................................1
Chapter 3 - Growth of Functions ......................................................................................................................2
Chapter 6 - Heapsort .........................................................................................................................................2
Chapter 7 - Quicksort ........................................................................................................................................3
Chapter 8 - Sorting in Linear Time ...................................................................................................................3
Chapter 10 - Elementary Data Structures .........................................................................................................5
Chapter 11 - Hash Tables ..................................................................................................................................6
Chapter 12 - Binary Search Trees .....................................................................................................................8
Chapter 13 - AVL Trees .....................................................................................................................................9
Chapter 15 - Dynamic Programming ..............................................................................................................10
Chapter 16 - Greedy Algorithms .....................................................................................................................11
Chapter x - Remaining Examples & Dijkstra’s Algorithm .............................................................................11
Chapter 1 - The Role of Algorithms in Computing
• An algorithm is a computational procedure that takes input and produces an output. It solves a
computational problem and is correct if, for every input instance it produces the right output.
• Not every problem solved by algorithms has an easily identified set of candidate solutions.
• A data structure is a way to store and organize data in order to facilitate access and modification.
• It is a given to choose an algorithm wisely, such that it will be time and space efficient.
Chapter 2 - Getting Started
➤2.1 Insertion Sort
• Insertion sort is a sorting algorithm that sorts by inserting an unsorted
element to the already sorted array. It is an efficient algorithm for
sorting a small number of elements.
• It sorts the input in place, it rearranges the elements, keys, within the
array, though it makes use of a subarray to store the still unsorted elements in.
• Insertion sort runs on Θ(n2) in average and worst case scenarios and has a best case scenario in Θ(n).
➤2.2 Analyzing Algorithms
• The running time of the algorithm grows with the input size. The running time is the amount of steps
executed to get an output.
• Assuming each line takes the same time, each execution of the ith line takes time ci, where ci is a
constant cost. A statement that takes ci steps to execute an executes n times will contribute cin to the total
running time.
• The worst case running time is always taken into consideration because it guarantees that the algorithm
will never take any longer; plus, for some algorithms they actually occur fairly often.
• For the rate of growth of the running time, only the leading term is taken. I.e. 2 n2 will be just n2.
➤2.3 Designing Algorithms
2.3.1 The divide-and-conquer approach
• Many algorithms are recursive in structure. It follows a divide-and-
conquer approach where the problem is divided in smaller subproblems
that will be solved recursively. Then the results are combined.
• Merge sort uses such approach. The elements are divided into two
subparts, then the subparts are sorted recursively using merge sort.
• A key operation is the merging of two sorted sequences by calling
MERGE(A, p, q, r) where p ≤ q < r. This takes Θ(n) time.
• The sentinel card(∞) is there to say one
subsequence has no more elements, therefore, the
elements in the other subsequence will always be
bigger.
, 2.3.2 Analyzing divide-and-conquer algorithms
• When an algorithm contains a recursive call to itself, its running time is often described by a recursive
equation, a recurrence. It describes the overall running time on a problem of size n in terms of the
running time on smaller inputs than n.
• The running time on a problem of size n is T(n). If the size is small enough such that n ≤ c, then it takes
constant time which is Θ(1).
• Suppose division yields a subproblems that are 1/b the size of the original problem. It will take aT(n/b)
to solve all the subproblems. Next, the time it takes to divide, D(n), and the time to combine, C(n), also
need to be added to get the recurrence.
{aT (n /b) + D(n) + C(n)
Θ(1) if n ≤ c,
• T (n) =
otherwise
• While looking at merge sort and n > 1, the running time is broken down as follows:
- Dividing is done in constant time, D(n) = Θ(1).
- Recursively solving 2 subproblems each of size n/2 will add 2T(n/2) to the running time.
- Merging the subproblems takes Θ(n) as stated before, so C(n) = Θ(n)
• Which makes the worst case running time for merge sort: 2T(n/2) + Θ(n) if n > 1. If n = 1, Θ(1).
• For merge sort, T(n) = Θ(n lg n), as the recurrence if n > 1 can be rewritten as T(n) = 2T(n/2) + cn
where constant c represents the time required to solve problems of size 1 as well as the time per array
element of the divide and combine steps.
Chapter 3 - Growth of Functions
➤3.1 Asymptotic Notation
• The asymptotic efficiency of algorithms is studied when the size of the input is large enough to make
only the order of growth of the running time relevant.
• Asymptotic notations are well suited to characterizing running times no matter what the input is.
• The Θ-notation is asymptotic tight bound (=), with tight bound meaning the running time is nailed
within a constant factor above and below.
• The O-notation is asymptotic upper bound (≤), this is used when a running time is only bound from
above. So in this case the ‘big-o’ of a running time can actually be much larger than in reality.
• The Ω-notation is asymptotic lower bound (≥), it says that an algorithm takes at least a certain amount
of time. It bounds the growth of the running time from below.
• Using asymptotic notation in equations can help eliminate inessential detail and clutter.
• The right-hand side of an equation provides less detail than the left-hand side.
• The o-notation is asymptotic upper bound not tight (<), e.g. 2n = o(n2) but 2n2 ≠ o(n2).
• The ω-notation is asymptotic lower bound not tight (>), e.g. n2/2 = ω(n) but n2/2 ≠ ω(n2).
Chapter 6 - Heapsort
➤6.1 Heaps
• The (binary) heap data structure is an array object that can be viewed as a nearly complete binary tree.
• Each node of the tree corresponds to an element of the array.
• An array A that represents a heap has two attributes, A.length which gives the number of elements of the
array and A.heap-size which represents how many elements in the heap are stored in A.
• In a max-heap, which is used for heapsort, the max-heap property is that A[parent(i)] ≥ A[i]. So the
largest element will be stored at the root. In a min-heap it is the opposite.
• The height of a node is the maximal length of a path to a leaf(a node without children). The height of the
tree is the height of the root. Its height is Θ(lg n).
➤6.2 Maintaining The Heap Property and Building A Max-Heap
• Maintaining the heap property is done with procedure MAX-HEAPIFY,
which lets the value at A[i] float down so that the largest element will
eventually become the root.
• By calling BUILD-MAX-HEAP, MAX-HEAPIFY is called
on all nodes such that the array now will be in order.
