> For the complete documentation index, see [llms.txt](https://beesec.gitbook.io/algorithms/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://beesec.gitbook.io/algorithms/strings/two-pointers/valid-mountain-array.md).

# Valid Mountain Array

{% hint style="info" %}
<https://leetcode.com/problems/valid-mountain-array/solution/>
{% endhint %}

{% tabs %}
{% tab title="Question" %}
Given an array of integers `arr`, return *`true` if and only if it is a valid mountain array*.

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

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

![](https://assets.leetcode.com/uploads/2019/10/20/hint_valid_mountain_array.png)

**Example 1:**

```
Input: arr = [2,1]
Output: false
```

{% endtab %}

{% tab title="Answer" %}
From the Leetcode solution:

**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.<br>

```python
class Solution(object):
    def validMountainArray(self, A):
        N = len(A)
        i = 0

        # walk up
        while i+1 < N and A[i] < A[i+1]:
            i += 1

        # peak can't be first or last
        if i == 0 or i == N-1:
            return False

        # walk down
        while i+1 < N and A[i] > A[i+1]:
            i += 1

        return i == N-1
```

From [here](https://leetcode.com/problems/valid-mountain-array/discuss/194900/C%2B%2BJavaPython-Climb-Mountain)

Two people climb from left and from right separately. If they are climbing the same mountain, they will meet at the same point.

```python
class Solution:
    def validMountainArray(self, arr: List[int]) -> bool:
        A = arr
        i, j, n = 0, len(A) - 1, len(A)
        while i + 1 < n and A[i] < A[i + 1]: 
            i += 1
        while j > 0 and A[j - 1] > A[j]: 
            j -= 1
        return 0 < i == j < n - 1
```

{% endtab %}
{% endtabs %}