896. Monotonic Array

An array is monotonic if it is either monotone increasing or monotone decreasing.

An array nums is monotone increasing if for all i <= j, nums[i] <= nums[j]. An array nums is monotone decreasing if for all i <= j, nums[i] >= nums[j].

Given an integer array nums, return true if the given array is monotonic, or false otherwise.
 

Example 1:

Input: nums = [1,2,2,3]
Output: true

Example 2:

Input: nums = [6,5,4,4]
Output: true

Example 3:

Input: nums = [1,3,2]
Output: false

Constraints:

• $1<=nums.length<=10^5$
• $-10^5<=nums[i]<=10^5$

From: LeetCode
Link: 896. Monotonic Array

Solution:

Ideas:

• Track both possibilities: non-decreasing and non-increasing.

• Knock out a possibility when a violating pair appears.

• Early exit if both become false.

• Handles duplicates naturally (e.g., [2,2,2] is monotonic).

Code:

bool isMonotonic(int* nums, int numsSize) {
    if (numsSize <= 2) return true;

    bool nonDecreasing = true;
    bool nonIncreasing = true;

    for (int i = 1; i < numsSize; ++i) {
        if (nums[i] > nums[i - 1]) nonIncreasing = false;
        if (nums[i] < nums[i - 1]) nonDecreasing = false;
        if (!nonDecreasing && !nonIncreasing) return false; // early exit
    }
    return true;
}