# 941. Valid Mountain Array

### Problem statement

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] < arr[i]`
        
    * `arr[i] > arr[i + 1] > ... > arr[arr.length - 1]`
        

![941_hint_valid_mountain_array.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1666030036023/KryLA1mSU.png align="left")

#### Example 1

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

#### Example 2

```plaintext
Input: arr = [3,5,5]
Output: false
```

#### Example 3

```plaintext
Input: arr = [0,3,2,1]
Output: true
```

#### Constraints

* `1 <= arr.length <= 10^4`.
    
* `0 <= arr[i] <= 10^4`.
    

### Solution

Following the conditions, we have the following implementation.

#### Code

```cpp
#include <vector>
#include <iostream>
using namespace std;
bool validMountainArray(vector<int>& arr) {
    if (arr.size() < 3) {
        return false;
    }
    const int N = arr.size() - 1;
    int i = 0;
    while (i < N && arr[i] < arr[i + 1]) {
        i++;
    }
    if (i == 0 || i == N) {
        return false;
    }
    while (i < N && arr[i] > arr[i + 1]) {
        i++;
    }
    return i == N;
}
int main() {
    vector<int> arr{2,1};
    cout << validMountainArray(arr) << endl;
    arr = {3,5,5};   
    cout << validMountainArray(arr) << endl;
    arr = {0,3,2,1};   
    cout << validMountainArray(arr) << endl;
    arr = {9,8,7,6,5,4,3,2,1,0};
    cout << validMountainArray(arr) << endl;
}
```

```plaintext
Output:
0
0
1
0
```

#### Complexity

* Runtime: `O(N)`, where `N = arr.length`.
    
* Extra space: `O(1)`.
    

### References

* [https://leetcode.com/problems/valid-mountain-array/](https://leetcode.com/problems/valid-mountain-array/)
    

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