An array is said to be monotonic if its elements, from left to right, are entirely no-increasing or enteriley non-decreasing.

isMonotonic.cpp

using namespace std;

// O(n) time | O(i) space -where n is the length of the arrat
bool isMonotonic(vector<int> array) {
	if (array.size() < 2) return true;

	int is_non_increasing = true;
	int is_non_decreasing = true;

	for (int i = 1; i < array.size(); i++) {
		if (array[i] > array[i - 1]) { // increasing
			is_non_increasing = false;
		}
		if (array[i] < array[i - 1]) { // decreasing
			is_non_decreasing = false;
		}
	}
  return is_non_decreasing || is_non_increasing;
}