3. (20%) A Prfer sequence of length n- 2, where n > 2, is any sequence of integers between 1 and n with repetitions allowed. (a) Prove that there are n"-2 Prfer sequences of length n- 2. (5%) (b) Let K, denote a complete graph of n vertices (i.e., each pair of vertices is connected by an edge). Prove that there is a one-to-one correpsondce (bijection) between the set of spanning tree of Kn and the set of Prfer sequences of length n 2. (15%)