monotonic array

This problem was asked by Facebook.

A monotonic array is an array whose elements, from left to right, are entirely non increasing, or entirely non decreasing. Return true if the given array is monotonic.

Example
function monotonicArray(arr){

}

const arr = [-1, -5, -10, -1100, -1100, -1101, -1102, -9001]
console.log(monotonicArray(arr)) // true

Solution

function monotonicArray(arr) {
  let increasing = true,
    decreasing = true;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] > arr[i + 1]) {
      increasing = false;
    } else if (arr[i] < arr[i + 1]) {
      decreasing = false;
    }
  }
  return increasing || decreasing;
}

const arr = [-1, -5, -10, -1100, -1100, -1101, -1102, -9001];
console.log(monotonicArray(arr)); // true

- This checks the array in O(n) time complexity by going through the elements sequentially and keeping track if the elements are either entirely increasing or decreasing.