941. Valid Mountain Array-优快云博客

本文详细解析了LeetCode题目941的有效山脉数组问题，介绍了如何判断一个整数数组是否构成有效的山脉数组，并提供了一个Python解决方案。通过对数组长度、最大值位置及元素递增和递减特性的分析，实现了一个时间复杂度为O(N)、空间复杂度为O(1)的高效算法。

1. Question:

941. Valid Mountain Array

https://leetcode.com/problems/valid-mountain-array/

Given an array A of integers, return true if and only if it is a valid mountain array.

Recall that A is a mountain array if and only if:

A.length >= 3
There exists some i with 0 < i < A.length - 1 such that:
- A[0] < A[1] < ... A[i-1] < A[i]
- A[i] > A[i+1] > ... > A[B.length - 1]

Example 1:

Input: [2,1]
Output: false

Example 2:

Input: [3,5,5]
Output: false

Example 3:

Input: [0,3,2,1]
Output: true

Note:

0 <= A.length <= 10000
0 <= A[i] <= 10000

2. Solution:

class Solution:
    def validMountainArray(self, A):
        """
        :type A: List[int]
        :rtype: bool
        """

        size = len(A)

        if size < 3:
            return False

        max_value = max(A)
        max_index = A.index(max_value)
        
        if max_index == 0 or max_index == size-1:
            return False
        
        for i in range(1, max_index + 1):
            if A[i - 1] >= A[i]:
                return False
        

        for i in range(max_index, size - 1):
            if A[i] <= A[i + 1]:
                return False

        return True