JavaScript Coding Test Course, Finding the K-th Number

Hello! Today we will learn how to solve coding test problems with JavaScript. The topic of this tutorial is ‘Finding the K-th Number’. Through this problem, we will learn how to sort an array and find the value at a specific index. Let’s look at the problem statement and the solution process step by step.

Problem Description

You need to find the K-th number that satisfies certain conditions from the given array. The specific problem description is as follows.

Problem: Finding the K-th Number

Given an array and an integer K, you need to return the K-th smallest number after sorting the array in ascending order.

Input:
- First line: Integer N (length of the array), Integer K (position of the number to find)
- Second line: An array of N integers

Output:
- K-th smallest number

Example

  • Input: 5 2
  • Array: [3, 1, 2, 5, 4]
  • Output: 2

From the above input values, if we sort the array in ascending order, it becomes [1, 2, 3, 4, 5], and the 2nd number is 2.

Solution Process

The steps required to solve the problem are as follows.

  1. Read the input values and set the array and K.
  2. Sort the array in ascending order.
  3. Output the value at the K-th index.

Step 1: Read Input Values

In JavaScript, you can use the prompt function to receive input values. However, coding test platforms usually read values through standard input. Here, we will directly declare the array for testing.


const arr = [3, 1, 2, 5, 4];
const K = 2; // K-th number

Step 2: Sort the Array

In JavaScript, you can use the sort method to sort an array. This method performs string sorting by default, so you need to provide a callback function for number sorting.


arr.sort((a, b) => a - b);

The above code sorts the array in ascending order, meaning it arranges from the smallest number to the largest number.

Step 3: Return the K-th Number

Since array indices start at 0, to get the K-th number, you need to use K-1 as the index. Therefore, you can do the following.


const kthNumber = arr[K - 1];
console.log(kthNumber); // Output

Complete Code

Now, let’s combine all the steps and write the complete code.


function findKthNumber(arr, K) {
    // Sort the array in ascending order
    arr.sort((a, b) => a - b);
    
    // Return the K-th number (K is 1-based index, so use K-1)
    return arr[K - 1];
}

// Test
const arr = [3, 1, 2, 5, 4]; // Example array
const K = 2; // K-th number
const result = findKthNumber(arr, K);
console.log(result); // Output result: 2

Conclusion

In this tutorial, we solved the ‘Finding the K-th Number’ problem. We learned how to use the JavaScript array method sort to sort an array and find the value at a specific position. When solving algorithm problems, it is important to understand the problem well and break it down into smaller units. Next time, we will come back with more diverse problems. Thank you!