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

Note:

1. 0 <= A.length <= 10000
2. 0 <= A[i] <= 10000 

Approach #1: 

class Solution {
public:
    bool validMountainArray(vector<int>& A) {
        int len = A.size();
        if (len < 3) return false;
        auto index = max_element(A.begin(), A.end()) - A.begin();
        if (index == len - 1 || index == 0) return false; 
        for (int i = 0; i < index; ++i) 
            if (A[i] >= A[i+1]) return false;
        for (int i = index; i < len-1; ++i)
            if (A[i] <= A[i+1]) return false;
        return true;
    }
};