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package ProjectEuler;

/**

* The sum of the squares of the first ten natural numbers is, 1^2 + 2^2 + ... + 10^2 = 385 The

* square of the sum of the first ten natural numbers is, (1 + 2 + ... + 10)^2 = 552 = 3025 Hence

* the difference between the sum of the squares of the first ten natural numbers and the square of

* the sum is 3025 − 385 = 2640. Find the difference between the sum of the squares of the first N

* natural numbers and the square of the sum.

*

* <p>link: https://projecteuler.net/problem=6

*/

public class Problem06 {

public static void main(String[] args) {

int[][] testNumbers = {

{10, 2640},

{15, 13160},

{20, 41230},

{50, 1582700}

};

for (int[] testNumber : testNumbers) {

assert solution1(testNumber[0]) == testNumber[1]

&& solutions2(testNumber[0]) == testNumber[1];

}

}

private static int solution1(int n) {

int sum1 = 0;

int sum2 = 0;

for (int i = 1; i <= n; ++i) {

sum1 += i * i;

sum2 += i;

}

return sum2 * sum2 - sum1;

}

private static int solutions2(int n) {

int sumOfSquares = n * (n + 1) * (2 * n + 1) / 6;

int squareOfSum = (int) Math.pow((n * (n + 1) / 2.0), 2);

return squareOfSum - sumOfSquares;

}

}