C# Coding Test Course, Hacking Efficiently

Coding tests are an essential process that many software developers and engineers must undergo to pass the entry barrier. In this article, we will discuss how to solve algorithm problems commonly encountered in coding tests using C#. In particular, we will explore what an efficient hacking process is and the algorithm patterns required for it.

Problem: Sum of Two Numbers

Given an integer array and a target integer ‘target’, find the indices of the two numbers in the array that add up to the ‘target’. The indices start from 0, and each number must be used exactly once.

Example Input

nums = [2, 7, 11, 15]
target = 9

Example Output

[0, 1]

Problem Interpretation

The above problem is quite famous as the ‘Two Sum’ problem. You must choose two numbers from the given array such that their sum equals the provided target value. This problem can be approached in various ways, but here we will focus on an efficient approach.

Approach

1. **Brute Force**: This method checks all possible pairs in the array to see if their sum matches the ‘target’. However, this approach has a time complexity of O(n^2), making it very inefficient.

2. **Using a HashMap**: This method uses a hashmap in the form of a dictionary to store the complement of the current number (target – current number) that we are currently looking at. This approach can solve the problem with a time complexity of O(n).

Implementation

C# Code Implementation

using System;
using System.Collections.Generic;

public class Solution
{
    public int[] TwoSum(int[] nums, int target)
    {
        Dictionary map = new Dictionary();

        for (int i = 0; i < nums.Length; i++)
        {
            int complement = target - nums[i];
            if (map.ContainsKey(complement))
            {
                return new int[] { map[complement], i };
            }
            map[nums[i]] = i;
        }
        throw new ArgumentException("No two sum solution");
    }
}

Code Explanation

  • First, we create an empty hashmap (>). This map will store numbers as keys and their indices as values.
  • As we iterate through the array, we calculate the complement for each number and check if it already exists in the hashmap.
  • If it exists, we return the index of that complement and the current index.
  • If it doesn’t exist, we add the current number and index to the hashmap.
  • We repeat this process for all numbers in the array.

Execution Example

When executed, based on the given nums and target, the following result will be produced:

var solution = new Solution();
var result = solution.TwoSum(new int[] { 2, 7, 11, 15 }, 9);
Console.WriteLine(string.Join(", ", result)); // Output: 0, 1

Tips for Improving Efficiency

When selecting data structures and algorithms in coding tests, it is important to choose what is suitable for each case. Below are some tips to consider.

  • **Time Complexity**: Consider the execution time of the algorithm. It’s preferable to choose the fastest algorithm.
  • **Space Complexity**: Consider memory usage. Instead of using additional arrays or lists, it’s better to utilize existing ones.
  • **Vary Test Cases**: Test a variety of scenarios to enhance generality and stability.

Conclusion

Preparing for coding tests using C# revolves around understanding which algorithms solve problems most efficiently. In this lesson, we explained a basic technique using hashmaps through the ‘Two Sum’ problem. We hope you gain experience by solving various algorithm problems in the future.

Finally, remember that coding tests involve not just problem-solving skills but also the thought process of understanding the problem and finding effective solutions. These skills are very important in your journey as a developer.

References