Valid Mountain Array

Description

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[A.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:

1. 0 <= A.length <= 10000
2. 0 <= A[i] <= 10000 

 

 

Solution(javascript)

const validMountainArray = function (A = []) {
  let maxIndex = null
  for (let i = 1; i <= A.length - 1; i++) {
    if (maxIndex === null) {
      if (A[i] < A[i - 1]) {
        maxIndex = i - 1
      } else if (A[i] === A[i - 1]) {
        return false
      }
    } else if (A[i] >= A[i - 1]) {
      return false
    }
  }
  return maxIndex > 0 && maxIndex < A.length - 1
}