2894. Divisible and Non-divisible Sums Difference

2894. Divisible and Non-divisible Sums Difference

Difficulty: Easy

Topics: Math

You are given positive integers n and m.

Define two integers as follows:

• num1: The sum of all integers in the range [1, n] (both inclusive) that are not divisible by m.
• num2: The sum of all integers in the range [1, n] (both inclusive) that are divisible by m.

Return the integer num1 - num2.

Example 1:

• Input: n = 10, m = 3
• Output: 19
• Explanation: In the given example:
  • Integers in the range [1, 10] that are not divisible by 3 are [1,2,4,5,7,8,10], num1 is the sum of those integers = 37.
  • Integers in the range [1, 10] that are divisible by 3 are [3,6,9], num2 is the sum of those integers = 18.
  • We return 37 - 18 = 19 as the answer.

Example 2:

• Input: n = 5, m = 6
• Output: 15
• Explanation: In the given example:
  • Integers in the range [1, 5] that are not divisible by 6 are [1,2,3,4,5], num1 is the sum of those integers = 15.
  • Integers in the range [1, 5] that are divisible by 6 are [], num2 is the sum of those integers = 0.
  • We return 15 - 0 = 15 as the answer.

Example 3:

• Input: n = 5, m = 1
• Output: -15
• Explanation: In the given example:
  • Integers in the range [1, 5] that are not divisible by 1 are [], num1 is the sum of those integers = 0.
  • Integers in the range [1, 5] that are divisible by 1 are [1,2,3,4,5], num2 is the sum of those integers = 15.
  • We return 0 - 15 = -15 as the answer.

Constraints:

• 1 <= n, m <= 1000

Hint:

1. With arithmetic progression we know that the sum of integers in the range [1, n] is n * (n + 1) / 2 .

Solution:

We need to find the difference between the sum of integers in the range [1, n] that are not divisible by a given integer m and the sum of those that are divisible by m. The solution can be efficiently derived using arithmetic progression formulas.

Approach

1. Total Sum Calculation: The sum of all integers from 1 to n can be calculated using the formula for the sum of an arithmetic series: total_sum = (n x (n + 1))/2.
2. Sum of Divisible Numbers: The sum of integers divisible by m up to n forms an arithmetic sequence. The number of terms in this sequence is k = n/m. The sum of this sequence is given by sum2 = m x ((k x (k + 1))/2).
3. Difference Calculation: The difference between the sum of non-divisible numbers and the sum of divisible numbers can be derived as total_sum − 2 x sum2, since the total sum includes both divisible and non-divisible numbers.

Let's implement this solution in PHP: 2894. Divisible and Non-divisible Sums Difference

<?php
/**
 * @param Integer $n
 * @param Integer $m
 * @return Integer
 */
function differenceOfSums($n, $m) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Example cases
echo differenceOfSums(10, 3) . "\n"; // Output: 19
echo differenceOfSums(5, 6) . "\n";  // Output: 15
echo differenceOfSums(5, 1) . "\n";  // Output: -15
?>

Explanation:

1. Total Sum Calculation: Using the formula (n x (n + 1))/2, we compute the sum of all integers from 1 to n. This gives us the total sum of both divisible and non-divisible numbers.
2. Sum of Divisible Numbers: We determine the largest integer k such that k x m ≤ n. The sum of the first k terms of the sequence formed by multiples of m is calculated using the formula m x (k x (k + 1))/2.
3. Difference Calculation: By subtracting twice the sum of the divisible numbers from the total sum, we effectively compute the difference between the sum of non-divisible numbers and the sum of divisible numbers.

This approach efficiently computes the required difference using arithmetic progression formulas, ensuring optimal performance with a time complexity of O(1).

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