如何利用大O的定义，即 ”f(n) = O(g(n)) means there are positive constants c and k, such that 0 ≤ f(n) ≤ cg(n) for all n ≥ k. The values of c and k must be fixed for the function f and must not depend on n.” 或者反例来证明 f(n) 不等于 O(f(2n)) ?