What is the difference between Θ(n) and O(n)?

Question

Sometimes I see Θ(n) with the strange Θ symbol with something in the middle of it, and sometimes just O(n). Is it just laziness of typing because nobody knows how to type th

Answer 1

f(n) belongs to O(n) if exists positive k as f(n)<=k*n

f(n) belongs to Θ(n) if exists positive k1, k2 as k1*n<=f(n)<=k2*n

Wikipedia article on Big O Notation