941. Valid Mountain Array（easy） 

Given an array A of integers, return true if and only if it is a valid mountain array.

Recall that A is a mountain array if and only if:

• A.length >= 3
• There exists some i with 0 < i < A.length - 1
   such that:
  • A[0] < A[1] < ... A[i-1] < A[i]
  • A[i] > A[i+1] > ... > A[B.length - 1]

 

Example 1:

Input: [2,1]
Output: false

Example 2:

Input: [3,5,5]
Output: false

Example 3:

Input: [0,3,2,1]
Output: true

 1 straightforward but awkard

按照定義先找最大值的位置maxidx，maxidx如果=0或len-1，說明在首尾位置，則A為單調減／增，return False

否則把A分為maxidx的左右判斷是否單調增或減

class Solution:
    def validMountainArray(self, A):
        """
        :type A: List[int]
        :rtype: bool
        """
        if len(A) < 3: return False
        maxidx = A.index(max(A))
        if (maxidx == 0) or (maxidx == len(A) - 1): return False
        for i in range(0,maxidx):
            if (A[i+1] <=  A[i]):return False
        for i in range(maxidx+1,len(A)):
            if A[i] >= A[i-1]:return False
        return True