m-mujica

require 'date'

def is_valid_date? year, month, day
  return false if year < 2000 || year > 2999
  Date::valid_date?(year, month, day)
end

class Date

  def is_valid_date_for_millenium? millenium

m-mujica

A zero-indexed array A consisting of N integers is given.

An equilibrium index of this array is any integer P such that 0 ≤ P < N
and the sum of elements of lower indices is equal to the sum of elements of higher indices,

i.e. A[0] + A[1] + ... + A[P−1] = A[P+1] + ... + A[N−2] + A[N−1].

Sum of zero elements is assumed to be equal to 0. This can happen if P = 0 or if P = N−1.
For example, consider the following array A consisting of N = 7 elements:
A[0] = -7   A[1] =  1   A[2] = 5