#Given an array of positive numbers,find the maximum possible sum such that no two chosen numbers are adjacent
#Time-complexity: O(n),Auxiliary-space:O(1)


def max_sum(a)
   len=a.length
   incl=a[0]
   excl=0
   for i in 1...len
       temp=[excl,incl].max
       incl=excl+a[i]
       excl=temp
   end
   return [incl,excl].max
end

max_sum([1,2,3,-1,4,5,0]) # => 9