Practice/Oracle/Leetcode 845. Longest Mountain in Array

Leetcode 845. Longest Mountain in Array

CodingMust

Problem

You are given an integer array. Your task is to identify the longest contiguous subarray that forms a "mountain" pattern and return its length.

A mountain subarray is defined as a sequence where:

• The sequence has at least 3 elements
• Elements strictly increase up to some peak index
• After the peak, elements strictly decrease
• Both the ascending and descending portions must have at least one element each (excluding the peak)

If no valid mountain exists in the array, return 0.

Requirements

1. The mountain must be a contiguous subarray
2. Elements must strictly increase before the peak (no plateaus or equal adjacent elements)
3. Elements must strictly decrease after the peak (no plateaus or equal adjacent elements)
4. Minimum length requirement is 3 elements
5. Return the length of the longest mountain found, or 0 if none exists

Constraints

• 1 ≤ arr.length ≤ 10,000
• 0 ≤ arr[i] ≤ 1,000,000
• Aim for O(n) time complexity and O(1) space complexity

Examples

Example 1:

Input: arr = [2, 1, 4, 7, 3, 2, 5] Output: 5 Explanation: The subarray [1, 4, 7, 3, 2] forms a valid mountain. It increases from 1→4→7, then decreases 7→3→2.

Example 2:

Input: arr = [2, 2, 2] Output: 0 Explanation: All elements are equal, so no strictly increasing or decreasing sequence exists.

Example 3:

Input: arr = [0, 1, 2, 3, 4, 5] Output: 0 Explanation: The array only increases. A valid mountain requires both ascending and descending portions.

Example 4:

Input: arr = [1, 3, 2] Output: 3 Explanation: The entire array forms a mountain: 1→3 (up), then 3→2 (down).