23. Let $a_1, a_2, \ldots, a_n$ be a sequence of integers and assume that $a_0 = a_{n+1} = -\infty$. For any i with $1 \le i \le n$, we define $a_i$ to be a peak, if $a_i \ge \max(a_{i-1}, a_{i+1})$.

(a) (2%). Identify all peaks in the given sequence 6, 7, 4, 3, 2, 1, 4, 5.

(b) (2%). Write down a procedure that finds a peak for $a_1, \ldots, a_n$ in $O(\log n)$ time.

(c) (2%). Briefly justify your answer in (b).