Data Structures and Algorithms: cram pack

Formula sheets, the key method steps for every topic and 6 practice problems, with full solutions at the back.

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1Formulas and methods

Graph theory

Handshake lemma
Σ deg(v) = 2|E|
Complete graphs
|E(Kn)| = n(n − 1)/2, |E(Km,n)| = m·n
Regular graph
|E| = n·k/2
Tree and forest
|E| = n − 1, |E| = n − c
Leaves of a tree
L = 2 + Σ (deg(v) − 2) over deg(v) ≥ 3
Euler
circuit: all degrees even; path: exactly 2 odd degrees
Hamilton circuits in Kn
(n − 1)!/2
Dijkstra update
d(v) = min(d(v), d(u) + w(u, v))
Walks of length L
(Mᴸ)ij

Method

  1. Write down the degree of every vertex; the degrees add up to twice the number of edges.
  2. For Euler questions, check that the graph is connected and count the vertices with odd degree.
  3. For trees, use n − 1 edges and the handshake lemma.
  4. For Dijkstra, keep a table of tentative distances and settle the smallest unsettled vertex each round.
  5. For a minimum spanning tree, sort the edges by weight and skip every edge that closes a cycle (Kruskal), or grow one tree from a start vertex (Prim).

2Practice problems

1.Algorithms & complexityMedium

What is the running time of LSD radix sort on n integers with d digits in base k?

  1. AO(dn log k)
  2. BO(d(n + k))
  3. CO(n²)
  4. DO(n log n)
2.Algorithms & complexityHard

What is the difference between an undecidable problem and an intractable one?

  1. AUndecidable problems are in NP, while intractable problems are in P
  2. BBoth mean the problem has no algorithm at all
  3. CIntractable problems cannot be solved by any algorithm, while undecidable ones only need exponential time
  4. DAn undecidable problem has no algorithm for all inputs; an intractable one has one, but it is too slow
3.Data structuresMedium

What is the purpose of a sentinel (dummy) node in a linked list?

  1. ATo simplify code by removing special cases at the head or tail
  2. BTo prevent the list from growing beyond a fixed maximum number of nodes
  3. CTo make the list sorted
  4. DTo store the number of elements
4.Data structuresHard

A segment tree is typically used to:

  1. AAnswer range queries (such as range sums) and updates in O(log n)
  2. BSort strings alphabetically by storing each prefix in a separate tree node
  3. CStore a graph's edges
  4. DImplement a stack
5.Graph theoryMedium

A connected graph has 12 vertices and 13 edges. How many edges k must be removed to leave a spanning tree?

|V| = 12|E| = 13
k =
6.Graph theoryHard
411382121314ABCDEF

Dijkstra’s algorithm runs from A. Click the vertex that is settled (made permanent) in round 5; A itself is round 1.

start: Around 5
  1. 1A
  2. 2B
  3. 3C
  4. 4D
  5. 5E
  6. 6F

3Solutions

1.Algorithms & complexity

Answer: B · O(d(n + k))

Why this is the answer

Each of the d passes is a counting sort costing O(n + k), so for fixed d and k the total is linear in n.

Answer

B · O(d(n + k))
2.Algorithms & complexity

Answer: D · An undecidable problem has no algorithm for all inputs; an intractable one has one, but it is too slow

Why this is the answer

The halting problem has no algorithm at all; an NP-hard problem such as TSP can be solved by brute force, just not in known polynomial time.

Answer

D · An undecidable problem has no algorithm for all inputs; an intractable one has one, but it is too slow
3.Data structures

Answer: A · To simplify code by removing special cases at the head or tail

Why this is the answer

With a sentinel, every real node has a predecessor, so inserting or deleting at the head needs no special handling.

Answer

A · To simplify code by removing special cases at the head or tail
4.Data structures

Answer: A · Answer range queries (such as range sums) and updates in O(log n)

Why this is the answer

A segment tree stores aggregates for intervals, supporting range queries and point updates in O(log n) time.

Answer

A · Answer range queries (such as range sums) and updates in O(log n)
5.Graph theory

Answer: k = 2

Euler, Hamilton and trees: a connected graph has an Euler circuit (every edge once, back to the start) when every vertex has even degree, and an Euler path that is not a circuit when exactly two vertices have odd degree; the path then runs from one odd vertex to the other. A Hamilton circuit visits every vertex once; Kn has (n − 1)!/2 of them. A tree on n vertices has n − 1 edges, and a connected graph with m edges keeps a spanning tree after removing m − n + 1 edges.

Edges of a spanning tree

12 − 1 = 11

Edges to remove

k = 13 − 11 = 2

Answer

k = 2
6.Graph theory

Answer: D

Shortest paths and spanning trees: Dijkstra’s algorithm settles one vertex per round: the unsettled vertex with the smallest tentative distance. Its distance is then final, and every neighbour v gets d(v) = min(d(v), d(u) + w(u, v)). Kruskal’s algorithm builds a minimum spanning tree by taking the edges from light to heavy and skipping any edge that would close a cycle; Prim’s algorithm grows one tree from a start vertex by always adding the lightest edge that leaves it. A spanning tree of n vertices has n − 1 edges.

Dijkstra round by round

1. A (0): B: 4, C: 1
2. C (1): E: 9
3. B (4): D: 17
4. E (9): D: 11, F: 22
5. D (11)
6. F (22)

round 5

D

Answer

D

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