Longest Mountain in Array — LeetCode 845 Python Solution

The problem

You may recall that an array arr is a mountain array if and only if: arr.length >= 3 There exists some index i (0-indexed) with 0 < i < arr.length - 1 such that: arr[0] < arr[1] < ... < arr[i - 1] < arr[i] arr[i] > arr[i + 1] > ...

Example

Input

arr = [2,1,4,7,3,2,5]

Output

5

Explanation

The largest mountain is [1,4,7,3,2] which has length 5.

Python solution

class Solution:
    def longestMountain(self, arr: List[int]) -> int:
        n = len(arr)
        f = [1] * n
        g = [1] * n
        for i in range(1, n):
            if arr[i] > arr[i - 1]:
                f[i] = f[i - 1] + 1
        ans = 0
        for i in range(n - 2, -1, -1):
            if arr[i] > arr[i + 1]:
                g[i] = g[i + 1] + 1
                if f[i] > 1:
                    ans = max(ans, f[i] + g[i] - 1)
        return ans

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 845. Longest Mountain in Array is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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