Kotlin Coding Test, Finding the Sum of Consecutive Natural Numbers

Problem Description

The problem of finding the sum of consecutive natural numbers is a common topic in coding tests.
Given the input integer N, the problem is to find M such that the sum of consecutive natural numbers from 1 to M equals N. Here, M must be a natural number greater than or equal to 1, and the sum of consecutive natural numbers can be calculated using the following formula:

S = 1 + 2 + 3 + ... + M = M * (M + 1) / 2

Input

– Integer N (1 ≤ N ≤ 1,000,000) – the sum of natural numbers to be found

Output

– Output M when the sum of consecutive natural numbers from 1 to M equals N.
– If M does not exist, output -1.

Approach to Problem

To solve this problem, it is useful to first rewrite the condition S = N into an equation.
When setting S = 1 + 2 + ... + M equal to N, rearranging gives us:

N = M * (M + 1) / 2

The above equation can be transformed to 2N = M * (M + 1).
We can then solve for M using this equation and check if the value is a natural number.

Solution Process

1. To find possible M for N, start with M equal to 1 and increase it while calculating S.
2. If S equals N, print M and terminate.
3. If S exceeds N, there is no need to search further, print -1 and terminate.

Code Implementation

Below is the code that implements the above logic in Kotlin.

fun findContinuousNaturalSum(N: Int): Int {
        var sum = 0
        for (M in 1..N) {
            sum += M
            if (sum == N) {
                return M
            } else if (sum > N) {
                return -1
            }
        }
        return -1
    }

    fun main() {
        val N = 15
        val result = findContinuousNaturalSum(N)
        println("The value of M such that the sum of consecutive natural numbers equals $N is: $result")
    }

Code Explanation

– The findContinuousNaturalSum function finds M such that the sum of consecutive natural numbers equals
N.
– The sum variable stores the sum from 1 to M.
– Using a for loop, M is incremented from 1 to N, while calculating sum.
– If sum equals N, M is returned, and if sum exceeds N, -1 is returned to terminate.

Examples

Input: 15
Output: 5
Explanation: 1+2+3+4+5 = 15, so M is 5.

Input: 14
Output: -1
Explanation: There is no case where the sum from 1 to M equals 14.

Conclusion

The problem of finding the sum of consecutive natural numbers is an important one that effectively utilizes basic loops and conditional statements in programming.
Through this problem, you can enhance your understanding of basic syntax and logic construction in Kotlin, which is beneficial for learning approaches to algorithmic problem-solving.
It is recommended to practice these fundamental problems sufficiently to solve a variety of future problems.