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 

 本題邏輯比較簡單，但邊界條件較多，以致提交出現了不少問題。在大小反轉時，應注意下標不要增加，同時規避單調遞增與遞減的情況。本題Accepted程式碼如下：

class Solution {
    public boolean validMountainArray(int[] A) {
        int l = A.length;
        if (l < 3) {
            return false;
        }
        int flag = 0;
        for (int i = 0; i < l - 1; i++) {
            if (flag == 0) {
                if (A[i] >= A[i + 1]) {
                    if (i == 0) {
                        return false;
                    }
                    flag = 1;
                    i--;
                }
            } else {
                if (A[i] <= A[i + 1]) {
                    return false;
                }
            }
        }
        if (flag == 0) {
            return false;
        }
        return true;
    }
}