How do you solve Valid Mountain Array on LeetCode?

Valid Mountain Array (LeetCode #941) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(1) space complexity. Key insight: A valid mountain array must have a peak that is neither the first nor the last element.

What is the brute force solution for Valid Mountain Array?

The brute force approach for Valid Mountain Array is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute-force approach checks every possible peak in the array to see if it forms a valid mountain. This involves iterating through the array multiple times to validate the increasing and decreasing conditions. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Valid Mountain Array?

Valid Mountain Array uses the following concepts: Array. The recommended patterns to study are: Two Pointers, Array.

What are the interview tips for Valid Mountain Array?

Always clarify the problem constraints and edge cases before diving into the solution. Think about the properties of the array (increasing/decreasing) and how you can track them efficiently. Practice explaining your thought process as you code; it helps in interviews.

What are common mistakes when solving Valid Mountain Array?

Not checking if the peak is at the first or last index. Confusing equal elements with strictly increasing or decreasing conditions.

#941

Valid Mountain Array

Easy

ArrayTwo PointersArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(1)

💡

Intuition

Time O(n)Space O(1)

The optimal solution uses a single pass through the array to check for the mountain conditions, making it more efficient. We can track the increasing and decreasing phases with a simple loop.

⚙️

Algorithm

4 steps

1Step 1: Check if the length of the array is less than 3; if so, return false.
2Step 2: Initialize two pointers: one for the increasing phase and one for the decreasing phase.
3Step 3: Move through the array to find the peak, ensuring the values are strictly increasing until the peak is found.
4Step 4: Continue moving through the array to ensure the values are strictly decreasing after the peak.

solution.py15 lines

1# Full working Python code
2
3def validMountainArray(arr):
4    n = len(arr)
5    if n < 3:
6        return False
7    i = 0
8    while i + 1 < n and arr[i] < arr[i + 1]:
9        i += 1
10    if i == 0 or i == n - 1:
11        return False
12    while i + 1 < n and arr[i] > arr[i + 1]:
13        i += 1
14    return i == n - 1
15

ℹ

Complexity note: The complexity is O(n) because we only traverse the array once, checking for the increasing and decreasing conditions in a single pass.

1A valid mountain array must have a peak that is neither the first nor the last element.
2The array must strictly increase to the peak and strictly decrease after the peak.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.