# 941. Valid Mountain Array

## Description

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]

Example 1:

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


Example 2:

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


Example 3:

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


Constraints:

• 1 <= arr.length <= 104
• 0 <= arr[i] <= 104

## Solutions

• class Solution {
public boolean validMountainArray(int[] arr) {
int n = arr.length;
if (n < 3) {
return false;
}
int l = 0, r = n - 1;
while (l + 1 < n - 1 && arr[l] < arr[l + 1]) {
++l;
}
while (r - 1 > 0 && arr[r] < arr[r - 1]) {
--r;
}
return l == r;
}
}

• class Solution {
public:
bool validMountainArray(vector<int>& arr) {
int n = arr.size();
if (n < 3) return 0;
int l = 0, r = n - 1;
while (l + 1 < n - 1 && arr[l] < arr[l + 1]) ++l;
while (r - 1 > 0 && arr[r] < arr[r - 1]) --r;
return l == r;
}
};

• class Solution:
def validMountainArray(self, arr: List[int]) -> bool:
n = len(arr)
if n < 3:
return False
l, r = 0, n - 1
while l + 1 < n - 1 and arr[l] < arr[l + 1]:
l += 1
while r - 1 > 0 and arr[r] < arr[r - 1]:
r -= 1
return l == r


• func validMountainArray(arr []int) bool {
n := len(arr)
if n < 3 {
return false
}
l, r := 0, n-1
for l+1 < n-1 && arr[l] < arr[l+1] {
l++
}
for r-1 > 0 && arr[r] < arr[r-1] {
r--
}
return l == r
}

• function validMountainArray(arr: number[]): boolean {
const n = arr.length;
if (n < 3) {
return false;
}
let [i, j] = [0, n - 1];
while (i + 1 < n - 1 && arr[i] < arr[i + 1]) {
i++;
}
while (j - 1 > 0 && arr[j] < arr[j - 1]) {
j--;
}
return i === j;
}