Python Coding Test Course, Finding the Sum of Consecutive Natural Numbers

Problem Description

The problem of finding the sum of consecutive natural numbers is to find the number of ways in which the sum of consecutive natural numbers k, k+1, k+2, ..., k+m equals a given integer N. Here, k is a natural number, and m is a non-negative integer.

For instance, when N = 15, it can be represented as follows:

• 1 + 2 + 3 + 4 + 5 = 15
• 4 + 5 + 6 = 15
• 7 + 8 = 15
• 15 = 15 (not a consecutive case)

Therefore, there are a total of 3 cases when N = 15.

Solution Process

To solve this problem, the following steps are necessary:

1. Understand the general formula for the sum of consecutive natural numbers.
2. Starting from k, adjust m to check results until reaching N.
3. Try all possible values of k to count the number of cases that satisfy the conditions.

1. General Formula for the Sum of Consecutive Natural Numbers

The sum of consecutive natural numbers can be expressed with the following mathematical formula:

S = k + (k + 1) + (k + 2) + ... + (k + m) = (m + 1) * k + (0 + 1 + 2 + ... + m)

Rearranging the above expression gives:

S = (m + 1) * k + (m * (m + 1)) / 2

We will adjust this expression to fit N.

2. Starting from k and Adjusting m

Now we need to find the necessary conditions by increasing the value of m while ensuring it does not exceed N based on the value of k. We continue this process until we find cases where S = N.

3. Trying All Possible k Values

In this process, to avoid various inefficient cases, the maximum value of k should be around the square root of N. This will help reduce the complexity of the algorithm.

Code Implementation

Below is a Python code example based on the above algorithm:

def count_consecutive_sum(N):
    count = 0
    # Setting the range for k values
    for k in range(1, N // 2 + 2):
        total = 0
        m = 0
        while total < N:
            total += k + m
            m += 1
        if total == N:
            count += 1
    return count

# Test
print(count_consecutive_sum(15))  # Output: 3

Code Explanation

The above code returns the number of ways to represent the given integer N as the sum of consecutive natural numbers. It iterates through values of k starting from 1 to N/2 + 2, incrementing m to calculate the total sum. If the total matches N, it increments the count.

Time Complexity Analysis

The outer loop in the above code has a time complexity of O(N), and the inner loop approximately iterates O(N) times for each k. Therefore, the overall time complexity of the algorithm can be O(N^2) in the worst case. However, by reducing the range of k, it can practically execute more efficiently.

Optimization

Optimization can be achieved by restricting the maximum value of k to the square root of N, thus obtaining more efficient performance. The code can be modified as follows:

def count_consecutive_sum_optimized(N):
    count = 0
    k = 1
    while k * (k + 1) // 2 < N:  # Until k's value is less than N
        # Calculating the sum of consecutive numbers
        total = N - (k * (k - 1) // 2)
        if total > 0 and total % k == 0:
            count += 1
        k += 1
    return count

# Test
print(count_consecutive_sum_optimized(15))  # Output: 3

Optimized Code Explanation

The optimized code above improves performance by setting the square root of N as the maximum value of k. Additionally, it calculates total within the loop to determine the conditions. Theoretically, this code operates with a time complexity close to O(sqrt(N)).