Category: C# Coding Test Tutorials

Problem Description

The placement of parentheses can change the calculation result of an expression. For example,
2 * 3 + 4 and (2 * 3) + 4 yield the same result, but
2 * (3 + 4) gives a different result.

The goal is to solve the problem of finding the possible minimum value based on the placement of parentheses in a given expression.
The expression consists of numbers and operators (+, -, *).

Problem Definition

Given an array of integers and operators, perform the task of appropriately placing parentheses to
produce the smallest result.
Specifically, when the expression takes the following form:

2 * 3 - 5 + 7

You need to find a way to minimize this expression using parentheses.

Problem Approach

A strategy to solve this problem is to use recursive exploration to consider all placements of parentheses.
Generate all combinations of placements and compute the result for each case to find the smallest value.

1. Simplifying the Problem

First, let’s express the following expression in an array format.
For example, 2 * 3 - 5 + 7 is transformed into the following structure:

[2, '*', 3, '-', 5, '+', 7]

2. Recursive Approach

To place parentheses controlling long expressions at each possible position,
we will use a recursive function to explore all cases.
The main steps are as follows:

• Recursively divide the expression and add parentheses.
• Calculate the result of each sub-expression.
• Update the minimum value among the calculated results.

3. Code Implementation

Below is an example code implemented in C#.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string expression = "2*3-5+7";
        int result = MinValue(expression);
        Console.WriteLine("Minimum Value: " + result);
    }

    static int MinValue(string expression)
    {
        var numbers = new List();
        var operators = new List();

        // Split the input string into numbers and operators.
        for (int i = 0; i < expression.Length; i++)
        {
            if (char.IsDigit(expression[i]))
            {
                int num = 0;
                while (i < expression.Length && char.IsDigit(expression[i]))
                {
                    num = num * 10 + (expression[i] - '0');
                    i++;
                }
                numbers.Add(num);
                i--; // Adjust i value
            }
            else
            {
                operators.Add(expression[i]);
            }
        }

        return CalculateMin(numbers, operators);
    }

    static int CalculateMin(List numbers, List operators)
    {
        // Base case: When only one number is left
        if (numbers.Count == 1)
            return numbers[0];

        int minValue = int.MaxValue;

        for (int i = 0; i < operators.Count; i++)
        {
            char op = operators[i];
            List leftNumbers = numbers.ToList();
            List rightNumbers = numbers.ToList();
            List leftOperators = operators.GetRange(0, i);
            List rightOperators = operators.GetRange(i + 1, operators.Count - i - 1);

            // Divide the left and right expressions based on the operator.
            int leftValue = CalculateMin(leftNumbers.GetRange(0, i + 1), leftOperators);
            int rightValue = CalculateMin(rightNumbers.GetRange(i + 1, rightNumbers.Count - i - 1), rightOperators);

            // Perform the operation.
            int result = PerformOperation(leftValue, rightValue, op);

            // Update the minimum value
            if (result < minValue)
                minValue = result;
        }

        return minValue;
    }

    static int PerformOperation(int left, int right, char op)
    {
        switch (op)
        {
            case '+':
                return left + right;
            case '-':
                return left - right;
            case '*':
                return left * right;
            default:
                throw new InvalidOperationException("Unsupported operator.");
        }
    }
}

4. Code Explanation

The functions used in the above code serve the following purposes:

• MinValue: Splits the given expression string into numbers and operators and prepares initial data for minimum value calculation.
• CalculateMin: Recursively calculates all possible sub-expressions and finds the minimum value.
• PerformOperation: Performs calculations using two numbers and an operator.

Through this structure, all combinations of parentheses placements are explored, and the minimum value among the calculated results is derived.

Conclusion

I hope this example problem has helped you understand the placement of parentheses and algorithmic approaches.
Based on this method and code, you can tackle various problems to find the minimum values of different expressions.
By always simplifying the problem and practicing recursive approaches, you can maximize your algorithmic thinking.

Additional Learning Resources

For a deeper understanding of algorithms, please refer to the following materials:

• GeeksforGeeks – Algorithm Fundamentals
• LeetCode – Algorithm Exercises
• HackerRank – Algorithms

Enhance your problem-solving skills for coding test preparation.

Problem Description

Write a function that takes a given integer N and returns a new integer formed by sorting its digits in descending order.

Input Conditions

• An integer N is given, which is between 0 and 1 billion (inclusive).

Output Conditions

• Return an integer formed by sorting the digits of integer N in descending order.

Example

Input Example

N = 118372

Output Example

873211

Approach to the Problem

The following steps are needed to solve this problem.

1. Convert the integer N to a string.
2. Store each digit in a list.
3. Sort the list in descending order.
4. Combine the sorted list elements back into a string and convert it to an integer.

C# Code Implementation

Below is the C# code to solve the given problem.

            
                using System;
                using System.Linq;

                public class Program
                {
                    public static void Main(string[] args)
                    {
                        int N = 118372;
                        Console.WriteLine(SortDigitsDescending(N));
                    }

                    public static int SortDigitsDescending(int n)
                    {
                        // Convert the integer to a string
                        var digits = n.ToString().ToCharArray();

                        // Sort the string in descending order
                        Array.Sort(digits);
                        Array.Reverse(digits);

                        // Convert the sorted string back to an integer
                        return int.Parse(new string(digits));
                    }
                }
            
        

Code Explanation

I will explain each step used in the above code.

1. Convert the integer to a string

n.ToString().ToCharArray() is used to convert the integer to a string. Then, using the ToCharArray() method, each digit is converted to a character array.

2. Sort the digits

Array.Sort(digits) is called to sort the digits in ascending order. Then, Array.Reverse(digits) is called to change it to descending order.

3. Convert to an integer

Finally, new string(digits) converts the character array to a string, and int.Parse is used to convert it to an integer.

Time Complexity

The time complexity of this algorithm is O(d log d), where d is the number of digits in the input integer. The time taken to sort the digits is O(d log d), while other operations take O(d).

Conclusion

In this course, we learned how to sort a given integer in descending order. We effectively solved this problem using basic array manipulation and string conversion in C#. Although it is a fundamental algorithm problem, it can evolve into various application problems, so practicing more problems is essential.

Introduction

Hello! In this tutorial, we will solve algorithm problems using stacks and queues with C#.
Stacks and queues are some of the most basic and important data structures in computer science, widely used to solve various algorithm problems.
Through this tutorial, we hope you gain a solid understanding of the basic concepts of stacks and queues and deepen your understanding by solving problems frequently asked in coding tests.

Basic Concepts of Stack and Queue

A stack follows the Last In First Out (LIFO) principle, where the last entered data is the first to exit.
A queue follows the First In First Out (FIFO) principle, where the first entered data is the first to exit.
These two structures are essential for solving various programming problems.

Problem: Checking Parenthesis Balance

Problem Description: Write a function to check if the same types of parentheses in a given string are correctly opened and closed.
Examples of correct parentheses are “()[]{}”, and examples of incorrect parentheses are “(]”, “([)]”.

Input

• String s (1 <= s.length <= 100) – consists of lowercase and uppercase letters, numbers, and parentheses.

Output

• Returns true if all parentheses are correctly opened and closed; otherwise, returns false.

Examples

    Input: s = "()"
    Output: true

    Input: s = "([)]"
    Output: false

Solution Process

We will use a stack data structure to solve this problem. We will push open parentheses to the stack and, when encountering a closed parenthesis,
compare it with the top element of the stack to check if it matches correctly.
The process is as follows:

1. Create a map to store pairs of parentheses. For example, define it as { ‘)’: ‘(‘, ‘]’: ‘[‘, ‘}’: ‘{‘ }.
2. Initialize a stack.
3. Iterate through each character in the string.
4. If the current character is an open parenthesis, push it to the stack.
5. If it is a closed parenthesis, check if the stack is empty, and if not, verify if it matches the top element of the stack.
6. After iterating through the entire string, return true if the stack is empty, otherwise false.

C# Code Implementation

    using System;
    using System.Collections.Generic;

    public class Solution
    {
        public bool IsValid(string s)
        {
            // Dictionary to store parenthesis pairs
            Dictionary<char, char=""> parentheses = new Dictionary<char, char="">()
            {
                { ')', '(' },
                { ']', '[' },
                { '}', '{' }
            };

            Stack stack = new Stack(); // Initialize stack

            foreach (char c in s)
            {
                if (parentheses.ContainsKey(c)) // Check if it's a closing parenthesis
                {
                    // Return false if stack is empty or top element doesn't match
                    if (stack.Count == 0 || stack.Pop() != parentheses[c])
                    {
                        return false;
                    }
                }
                else // If it's an opening parenthesis
                {
                    stack.Push(c); // Push to stack
                }
            }

            return stack.Count == 0; // Return true if stack is empty
        }
    }
    </char,></char,>

Code Explanation

The above code defines the function `IsValid` that checks the balance of parentheses in the string `s`.
It first defines pairs of parentheses, initializes a stack, and then iterates through the input string, pushing open parentheses to the stack
and checking the top element for a match if it encounters a closing parenthesis.
If the stack is empty after checking all characters, it returns true, indicating all parentheses are correctly opened and closed.

Additional Examples

Example 1

    Input: s = "{[]}"
    Output: true

Explanation: Starts with ‘{‘ and ends with ‘}’, with ‘[‘ and ‘]’ correctly matched in between.

Example 2

    Input: s = "({[})"
    Output: false

Explanation: ‘[‘ appears right after ‘(‘ without a matching pair, so it’s incorrect.

Review Problem

In this tutorial, we solved a parenthesis balance problem using a stack.
I hope you now have a better understanding of stacks and queues.
Another problem to try next is “Implementing Queue using Stack.”
This is a great way to dive deeper into the basic concepts of stacks and their applications.
I recommend implementing and writing code on your own!

Conclusion

Stacks and queues are very important data structures in algorithms and programming.
There are many types of problems that can be solved by using these two data structures.
I hope this tutorial has been helpful in solving future programming problems!
Continue studying the applications of stacks and queues.