Monotonic Array

Monotonic Array

Write a function that takes in an array of integers and returns a boolean representing whether the array is monotonic.

An array is said to be monotonic if its elements, from left to right, are entirely non-increasing or entirely non-decreasing.

Non-increasing elements aren't necessarily exclusively decreasing; they simply don't increase. Similarly, non-decreasing elements aren't necessarily exclusively increasing; they simply don't decrease.

Note that empty arrays and arrays of one element are monotonic.

Sample Input

array = [-1, -5, -10, -1100, -1100, -1101, -1102, -9001]

Sample Output

true

Solution 1

// O(n) time | O(1) space
function isMonotonic(array) {
  if (array.length <= 2) return true;
  let direction = array[1] - array[0];
  for (let i = 2; i < array.length; i++) {
    if (direction === 0) {
      direction = array[i] - array[i - 1];
      continue;
    }
    if (breaksDirection(direction, array[i - 1] , array[i])) {
      return false;
    }
  }
  return true;
}

function breaksDirection(direction, previousInt, currentInt) {
  const difference = currentInt - previousInt;
  if (direction > 0) return difference < 0;
  return difference > 0;
}

// Do not edit the line below.
exports.isMonotonic = isMonotonic;

Solution 2

// O(n) time | O(1) space
function isMonotonic(array) {
  let isNonDecreasing = true;
  let isNonIncreasing = true;
  for (let i = 1; i < array.length; i++) {
    if (array[i] < array[i - 1]) isNonDecreasing = false;
    if (array[i] > array[i - 1]) isNonIncreasing = false;
  }
  return isNonDecreasing || isNonIncreasing
}

// Do not edit the line below.
exports.isMonotonic = isMonotonic;