Valid Mountain Array

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Leetcode

Medium

Arrays

Determine whether an array rises strictly to a single peak and then falls strictly, forming a mountain shape.

Acceptance 0%

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Problem Statement

Given an integer array arr, determine whether it forms a valid mountain array.

An array is a valid mountain array if:

its length is at least 3,
there exists an index peak with 0 < peak < n - 1,
the values strictly increase from arr[0] to arr[peak], and
the values strictly decrease from arr[peak] to arr[n - 1].

Return true if the array is a valid mountain array, otherwise return false.

A valid mountain must have exactly one peak and no plateaus.

Input Format

An integer array arr.

Output Format

Return true if arr is a valid mountain array; otherwise return false.

Constraints

$3 \le n$
The array must have exactly one peak.
Adjacent values must be strictly increasing before the peak and strictly decreasing after the peak.

Examples

Sample cases returned by the problem API.

Example 1

Input

arr = [0,3,2,1]

Output

true

Explanation

The array strictly increases from 0 to 3, then strictly decreases from 3 to 1, with a single peak at index 1.

Example 2

Input

arr = [3,5,5]

Output

false

Explanation

The values do not strictly increase and then strictly decrease because 5 is repeated, creating a plateau.

Show 1 more example

Example 3

Input

arr = [2,1]

Output

false

Explanation

The array is too short to form a mountain.

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