LeetCode 0941. Valid Mountain Array Solution in Java, C++, Python & More | Explanation + Code

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

Solution 1: Two Pointers

First, we check if the length of the array is less than 3. If it is, then it definitely is not a mountain array, so we return false directly.

Then, we use a pointer i to move from the left end of the array to the right, until we find a position i such that arr[i] > arr[i + 1]. After that, we use a pointer j to move from the right end of the array to the left, until we find a position j such that arr[j] > arr[j - 1]. If the condition i = j is satisfied, then it means that the array arr is a mountain array.

The time complexity is O(n), where n is the length of the array. The space complexity is O(1).

PythonJavaC++GoTypeScript

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

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

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

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

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; }(code-box)