Stacks and Queues
Processing Sequences of Elements
SoftUni Team
Technical Trainers
Software University
http://softuni.bg
Table of Contents
1. Stack<T> (LIFO – last in, first out)

Push(), Pop(), Peek(), ToArray(), Contains() and Count
2. Queue<T> (FIFO – first in, first out)

Enqueue(), Dequeue(), Peek(),
ToArray(), Contains() and Count
2
Questions
sli.do
#CSharp-Advanced
3
2
10
5
Stack<T>
Overview
Stack – Abstract Data Type
 Stacks provide the following functionality:
 Pushing an element at the top of the stack
 Popping element from the top fo the stack
 Getting the topmost element without removing it
10
10
10
5
5
5
2
2
2
Push
Pop
Peek
5
Push() – Adds an element on top of the Stack
10
52
Stack<int>
Count:
2
0
3
1
6
Pop() – Returns the last element from the stack
and removes it
Stack<int>
2
Count:
2
1
3
10
5
7
Peek() – Returns the last element from the stack
without removing it
Stack<int>
Count:
1
5
8
Problem: Reversing Strings
 Create a program that:
 Reads an input string
 Reverses it using a Stack
Input
Output
I Love C#
#C evoL I
Stacks and Queues
seueuQ dna skcatS
Check your solution here: https://judge.softuni.bg/Contests/
9
Solution: Reversing Strings
var input = Console.ReadLine();
var stack = new Stack<char>();
foreach (var ch in input)
{
stack.Push(ch);
}
while (stack.Count != 0)
{
Console.Write(stack.Pop());
}
Console.WriteLine();
Count property
is used without
parentheses
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
10
Stack – Constructors
Stack<int> stack = new Stack<int>();
Stack<int> stack = new Stack<int>(12);
Specified initial capacity
of internal array
string[] values =
{ “Advanced", “OOP", “OOP Advanced" };
Stack<string> stack = new Stack<string>(values);
Copied elements from the
specified collection and
remains their order
11
Stack – Utility Methods
Stack<int> stack = new Stack<int>();
int count = stack.Count;
bool exists = stack.Contains(2);
int[] array = stack.ToArray();
stack.Clear();
stack.TrimExcess();
Retains order of
elements
Remove all
elements
Resize the
internal array
12
Stack<T> – Overview of all operations
132
10
15
-7
52
Stack<int>
Push()
Pop()
Count:
2
1
0
3
5
4
5
Peek()
13
Problem: Simple Calculator
 Implement a simple calculator that can evaluate simple
expressions (only addition and subtraction)
Input
Output
2 + 5 + 10 - 2 - 1
14
2 - 2 + 5
5
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
14
Solution: Simple Calculator
var input = Console.ReadLine();
var values = input.Split(' ');
var stack = new Stack<string>(values.Reverse());
while (stack.Count > 1)
{
int first = int.Parse(stack.Pop());
String op = stack.Pop();
int second = int.Parse(stack.Pop());
//TODO: Add switch for operation (look next slide)
}
Console.WriteLine(stack.Pop());
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
15
Solution: Simple Calculator (2)
switch (op)
{
case "+":
stack.Push((first + second).ToString()); break;
case "-":
stack.Push((first - second).ToString()); break;
}
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
16
Problem: Decimal To Binary Converter
 Create a converter which takes a decimal number and converts
it into a binary number.
Input
Output
10
1010
1024
10000000000
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
17
Solution: Decimal To Binary Converter
var decimalNumber = int.Parse(Console.ReadLine());
var stack = new Stack<int>();
//TODO: Check if number is zero
while (decimalNumber != 0)
{
stack.Push(decimalNumber % 2);
decimalNumber /= 2;
}
while (stack.Count != 0)
{
Console.Write(stack.Pop());
}
Console.WriteLine();
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
18
Problem: Matching Brackets
 We are given an arithmetical expression with brackets (with nesting)
 Goal: extract all sub-expressions in brackets
1 + (2 - (2 + 3) * 4 / (3 + 1)) * 5
(2 + 3)
(3 + 1)
(2 - (2 + 3) * 4 / (3 + 1))
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
Problem: Matching Brackets
//TODO: Read input from console
for (int i = 0; i < input.Length; i++)
{
char ch = input[i];
if (ch == '(')
stack.Push(i);
else if (ch == ')')
int startIndex = stack.Pop();
string contents = input.Substring
(startIndex, i – startIndex + 1);
Console.WriteLine(contents);
}
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
Working with Stacks
Exercises in class
2
10
5
Queue<T>
Overview
Queue – Abstract Data Type
 Queues provide the following functionality:
 Adding an element at the end of the queue
10
5
2
 Removing the first element from the queue
10
5
2
 Getting the first element of the queue without removing it
10
5
2
23
Enqueue() – Adds an element to the front of
the queue
Queue<int>
Count:
43
2
0
1
121
15
-3
5
24
Dequeue() – Returns the first element from the
queue and removes it
Count:
Queue<int>
121
15
-3
42
3
5
25
Peek() – Returns the first element from the queue
without removing it
Count:
Queue<int>
121
2
15
26
Problem: Hot Potato
 Children form a circle and pass a hot potato clockwise
 Every nth toss a child is removed until only one remains
 Upon removal the potato is passed forward
 Print the child that remains last
Input
Output
Mimi Pepi Toshko
2
Removed Pepi
Removed Mimi
Last is Toshko
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
27
Solution: Hot Potato
var children = Console.ReadLine().Split(' ');
var number = int.Parse(Console.ReadLine());
Queue<string> queue = new Queue<string>(children);
while (queue.Count != 1)
Copied elements from the
{
specified collection and
for (int i = 1; i < number; i++)
remains their order
{
queue.Enqueue(queue.Dequeue());
}
Console.WriteLine($"Removed {queue.Dequeue()}");
}
Console.WriteLine($"Last in {queue.Dequeue()}");
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
28
Queue – Utility Methods
Queue<int> queue = new Queue<int>();
int count = queue.Count;
bool exists = queue.Contains(2);
int[] array = queue.ToArray();
queue.Clear();
queue.TrimExcess();
Retains order of
elements
Remove all
elements
Resize the
internal array
29
Queue<T> – Overview of all operations
Count:
Queue<int>
121
15
-3
5
4
2
1
0
3
5
Enqueue()
Peek()
Dequeue()
30
Problem: Math Potato
 Rework the previous problem so that a child is removed only on
a prime cycle (cycles start from 1)
 If a cycle is prime, just print the child's name
Input
Output
Mimi Pepi Toshko
2
Removed Pepi
Prime Mimi
Prime Toshko
Removed Mimi
Last is Toshko
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
31
Solution: Math Potato
//TODO: Read input from console
int cycle = 1;
while (queue.Count != 1)
{
for (int i = 1; i < number; i++)
{
queue.Enqueue(queue.Dequeue());
}
//TODO: Check if cycle is Prime, look at next slide
cycle++;
}
Console.WriteLine($"Last in {queue.Dequeue()}");
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
32
Solution: Math Potato (2)
You need to write your
own tool for checking if
numbers is prime
if (PrimeTool.IsPrime(cycle))
{
Console.WriteLine($"Prime {queue.Peek()}");
}
else
{
Console.WriteLine($"Removed {queue.Dequeue()}");
}
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/386#0
33
Working with Queues
Exercises in class
Summary
 Stack<T>

LIFO data structure
 Queue<T>

FIFO data structure
35
Stacks and Queues
?
https://softuni.bg/courses/advanced-csharp
License
 This course (slides, examples, demos, videos, homework, etc.)
is licensed under the "Creative Commons AttributionNonCommercial-ShareAlike 4.0 International" license
 Attribution: this work may contain portions from

"Fundamentals of Computer Programming with C#" book by Svetlin Nakov & Co. under CC-BY-SA license

"C# Part I" course by Telerik Academy under CC-BY-NC-SA license

"C# Part II" course by Telerik Academy under CC-BY-NC-SA license
37
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