Solution
Approach 1: One Pass
Intuition
If we walk along the mountain from left to right, we have to move strictly up, then strictly down.
Algorithm
Let's walk up from left to right until we can't: that has to be the peak. We should ensure the peak is not the first or last element. Then, we walk down. If we reach the end, the array is valid, otherwise its not.
Complexity Analysis

Time Complexity: , where is the length of
A
. 
Space Complexity: .
Analysis written by: @awice.