Editorial for THICC '17 P2 - Molly and Product - Olympiads Online Judge

Editorial for THICC '17 P2 - Molly and Product

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Author: Kirito

For $30\%$ ~30\%~ of points, it is possible to loop all $\mathcal O(N^2)$ ~\mathcal O(N^2)~ pairs and find their sum.

Time Complexity: $\mathcal O(N^2)$ ~\mathcal O(N^2)~

For the remaining $70\%$ ~70\%~ of points, we have to do some basic math. Let $\Sigma$ ~\Sigma~ be the sum of all $N$ ~N~ elements. Notice that

$\displaystyle \Sigma^2 = \text{our desired result} + A_0^2 + A_1^2 + \dots + A_{N-1}^2$

Thus we can find the square of the sum of all elements, and subtract the sum of the squares of all elements.

Time Complexity: $\mathcal O(N)$ ~\mathcal O(N)~

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