941-valid-mountain-array.js

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

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

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

Example 1:
Input: arr = [2,1]
Output: false

Example 2:
Input: arr = [3,5,5]
Output: false

Example 3:
Input: arr = [0,3,2,1]
Output: true
*/
/**
 * @param {number[]} arr
 * @return {boolean}
 */
var validMountainArray = function (arr) {
  let N = arr.length;
  let i = 0;

  // uphill
  while (i + 1 < N && arr[i] < arr[i + 1]) {
    i++;
  }

  if (i == 0 || i == N - 1) {
    return false;
  }

  // downhill
  while (i + 1 < N && arr[i] > arr[i + 1]) {
    i++;
  }

  return i == N - 1;
};

const result = validMountainArray([0, 3, 2, 1]);
console.log(result);