# Project Euler #44: Pentagon numbers

# Project Euler #44: Pentagon numbers

[deleted] + 0 comments I guess we're not allowing negative values of n for P_n = n * (3n - 1)? Otherwise, the answer to the test case would also include 117, since P_-11 = 187 = 117 + 70 = P_9 + P_7.

acossack + 0 comments Output Format: Print the pentagonal numbers corresponding to the test case in sorted order, each in a new line.

Формат выходных данных: Вывести пятиугольные числа для заданного теста через пробел.

Russian version is wrong.

byungwoo + 0 comments Test Case #3 - Must remove duplicate values.

cnhnyu + 0 comments If you are using C++, make sure you are using long for all data type. Mixing long and int will give your wrong results.

The maximum number is 10^6, you can just build a Pentagon numbers map from 1 to 10^6. Then to test if Pn - Pn-k or Pn - Pn+k in this map, if either of them in the map, just print Pn.

After you build this map, you can get the results in O(1) time complexity.

god_blessme + 0 comments For everybody who are struggling with timeout problem.Dont go through the loop to check whether a given number is pentagonal number.Just check,

n=(sqrt(24*x+1)+1)/6; if x is a pentagonal number then n should be a natural number.

Hope this helps!!!

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