Monotonic Array

This question is asked by Facebook. Given an array nums, return whether or not its values are monotonically increasing or monotonically decreasing.

Note:

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

Example(s)

Example 1:

Input: nums = [1, 2, 3, 4, 4, 5]
Output: true
Explanation:
The array is monotonically increasing:
1 <= 2 <= 3 <= 4 <= 4 <= 5

Example 2:

Input: nums = [7, 6, 3]
Output: true
Explanation:
The array is monotonically decreasing:
7 >= 6 >= 3

Example 3:

Input: nums = [8, 4, 6]
Output: false
Explanation:
The array is neither increasing nor decreasing:
8 >= 4 (decreasing), but 4 <= 6 (increasing)

Example 4:

Input: nums = [1, 1, 1, 1]
Output: true
Explanation:
All values are equal, which satisfies both conditions:
1 <= 1 <= 1 <= 1 (increasing)
1 >= 1 >= 1 >= 1 (decreasing)

Solution

The solution uses two flags:

Check increasing: Track if array is non-decreasing
Check decreasing: Track if array is non-increasing
Return result: Return true if either condition is satisfied

JavaScript Solution
Python Solution

JavaScript Solution - Two Flags

/**
* Check if array is monotonic (increasing or decreasing)
* @param {number[]} nums - Input array
* @return {boolean} - True if array is monotonic
*/
function isMonotonic(nums) {
if (nums.length <= 1) return true;

let isIncreasing = true;
let isDecreasing = true;

// Check each pair of adjacent elements
for (let i = 1; i < nums.length; i++) {
  if (nums[i] > nums[i - 1]) {
    // Found an increase, not decreasing
    isDecreasing = false;
  } else if (nums[i] < nums[i - 1]) {
    // Found a decrease, not increasing
    isIncreasing = false;
  }
  // If nums[i] === nums[i-1], both flags remain unchanged
}

// Array is monotonic if it's either increasing or decreasing
return isIncreasing || isDecreasing;
}

// Test cases
console.log('Example 1:', isMonotonic([1, 2, 3, 4, 4, 5])); 
// true (increasing)

console.log('Example 2:', isMonotonic([7, 6, 3])); 
// true (decreasing)

console.log('Example 3:', isMonotonic([8, 4, 6])); 
// false (neither)

console.log('Example 4:', isMonotonic([1, 1, 1, 1])); 
// true (both)

console.log('Test 5:', isMonotonic([1])); 
// true

Output:

Click "Run Code" to execute the code and see the results.

Python Solution - Two Flags

from typing import List

def is_monotonic(nums: List[int]) -> bool:
  """
  Check if array is monotonic (increasing or decreasing)
  
  Args:
      nums: Input array
      
  Returns:
      bool: True if array is monotonic
  """
  if len(nums) <= 1:
      return True
  
  is_increasing = True
  is_decreasing = True
  
  # Check each pair of adjacent elements
  for i in range(1, len(nums)):
      if nums[i] > nums[i - 1]:
          # Found an increase, not decreasing
          is_decreasing = False
      elif nums[i] < nums[i - 1]:
          # Found a decrease, not increasing
          is_increasing = False
      # If nums[i] == nums[i-1], both flags remain unchanged
  
  # Array is monotonic if it's either increasing or decreasing
  return is_increasing or is_decreasing

# Test cases
print('Example 1:', is_monotonic([1, 2, 3, 4, 4, 5]))  
# True (increasing)

print('Example 2:', is_monotonic([7, 6, 3]))  
# True (decreasing)

print('Example 3:', is_monotonic([8, 4, 6]))  
# False (neither)

print('Example 4:', is_monotonic([1, 1, 1, 1]))  
# True (both)

print('Test 5:', is_monotonic([1]))  
# True

Loading Python runtime...

Output:

Click "Run Code" to execute the code and see the results.

Alternative Solution (Single Pass with Direction)

Here's a version that determines direction first:

JavaScript Direction
Python Direction

/**
 * Determine direction first, then verify
 */
function isMonotonicDirection(nums) {
  if (nums.length <= 1) return true;
  
  // Find first non-equal pair to determine direction
  let direction = 0; // 0 = unknown, 1 = increasing, -1 = decreasing
  
  for (let i = 1; i < nums.length; i++) {
    if (nums[i] !== nums[i - 1]) {
      direction = nums[i] > nums[i - 1] ? 1 : -1;
      break;
    }
  }
  
  // If all elements are equal, return true
  if (direction === 0) return true;
  
  // Verify all subsequent pairs follow the direction
  for (let i = 1; i < nums.length; i++) {
    if (direction === 1 && nums[i] < nums[i - 1]) {
      return false;
    }
    if (direction === -1 && nums[i] > nums[i - 1]) {
      return false;
    }
  }
  
  return true;
}

def is_monotonic_direction(nums: List[int]) -> bool:
    """
    Determine direction first, then verify
    """
    if len(nums) <= 1:
        return True
    
    # Find first non-equal pair to determine direction
    direction = 0  # 0 = unknown, 1 = increasing, -1 = decreasing
    
    for i in range(1, len(nums)):
        if nums[i] != nums[i - 1]:
            direction = 1 if nums[i] > nums[i - 1] else -1
            break
    
    # If all elements are equal, return True
    if direction == 0:
        return True
    
    # Verify all subsequent pairs follow the direction
    for i in range(1, len(nums)):
        if direction == 1 and nums[i] < nums[i - 1]:
            return False
        if direction == -1 and nums[i] > nums[i - 1]:
            return False
    
    return True

Complexity

Time Complexity: O(n) - Where n is the length of the array. We visit each element once.
Space Complexity: O(1) - Only using a constant amount of extra space.

Approach

The solution uses two flags:

Initialize flags:
- isIncreasing = true: Assume array is increasing
- isDecreasing = true: Assume array is decreasing
Check adjacent pairs:
- For each pair (nums[i-1], nums[i]):
  - If nums[i] > nums[i-1]: Set isDecreasing = false
  - If nums[i] < nums[i-1]: Set isIncreasing = false
  - If nums[i] === nums[i-1]: Both flags remain unchanged
Return result:
- Return true if either isIncreasing or isDecreasing is true
- This means the array is either non-decreasing or non-increasing

Key Insights

Two flags: Track both increasing and decreasing possibilities
Single pass: Check all pairs in one pass
Equal elements: Don't affect monotonicity (1 <= 1 is true)
Early termination: Can optimize to return false early if both flags become false
O(n) time: Must check all pairs

Step-by-Step Example

Let's trace through Example 1: nums = [1, 2, 3, 4, 4, 5]

Initial: isIncreasing = true, isDecreasing = true

i = 1: nums[1] = 2, nums[0] = 1
  2 > 1 → isDecreasing = false
  isIncreasing = true, isDecreasing = false

i = 2: nums[2] = 3, nums[1] = 2
  3 > 2 → isDecreasing = false
  isIncreasing = true, isDecreasing = false

i = 3: nums[3] = 4, nums[2] = 3
  4 > 3 → isDecreasing = false
  isIncreasing = true, isDecreasing = false

i = 4: nums[4] = 4, nums[3] = 4
  4 === 4 → no change
  isIncreasing = true, isDecreasing = false

i = 5: nums[5] = 5, nums[4] = 4
  5 > 4 → isDecreasing = false
  isIncreasing = true, isDecreasing = false

Result: isIncreasing || isDecreasing = true || false = true

Visual Representation

Example 1: nums = [1, 2, 3, 4, 4, 5]

Values:  1  2  3  4  4  5
         ↑  ↑  ↑  ↑  ↑  ↑
         All increasing or equal

Check: 1 <= 2 <= 3 <= 4 <= 4 <= 5 ✓
Result: true (monotonically increasing)

Example 2: nums = [7, 6, 3]

Values:  7  6  3
         ↑  ↑  ↑
         All decreasing

Check: 7 >= 6 >= 3 ✓
Result: true (monotonically decreasing)

Example 3: nums = [8, 4, 6]

Values:  8  4  6
         ↑  ↑  ↑
         Decrease then increase

Check: 8 >= 4 ✓, but 4 <= 6 ✗
Result: false (neither)

Edge Cases

Empty array: Return true (vacuously true)
Single element: Return true
All equal: Return true (satisfies both conditions)
Two elements: Check if they're equal or one direction
Strictly increasing: Works (no equal elements)
Strictly decreasing: Works (no equal elements)

Important Notes

Non-strict: Allows equal elements (1 <= 1 is true)
Either direction: Can be increasing OR decreasing
Single pass: Efficient O(n) solution
Two flags: Track both possibilities simultaneously
O(1) space: Only need two boolean flags

Optimization: Early Return

We can optimize to return early if both flags become false:

function isMonotonicOptimized(nums) {
  if (nums.length <= 1) return true;
  
  let isIncreasing = true;
  let isDecreasing = true;
  
  for (let i = 1; i < nums.length; i++) {
    if (nums[i] > nums[i - 1]) {
      isDecreasing = false;
    } else if (nums[i] < nums[i - 1]) {
      isIncreasing = false;
    }
    
    // Early return if both are false
    if (!isIncreasing && !isDecreasing) {
      return false;
    }
  }
  
  return isIncreasing || isDecreasing;
}

Monotonic Array: Same problem
Longest Monotonic Subarray: Different problem
Best Time to Buy and Sell Stock: Uses monotonic property
Trapping Rain Water: Different problem

Takeaways

Two flags track both increasing and decreasing
Single pass through the array
Non-strict allows equal adjacent elements
O(n) time is optimal (must check all pairs)
O(1) space for the algorithm

Example(s)​

Solution​

JavaScript Solution - Two Flags

Output:

Python Solution - Two Flags

Output:

Alternative Solution (Single Pass with Direction)​

Complexity​

Approach​

Key Insights​

Step-by-Step Example​

Visual Representation​

Edge Cases​

Important Notes​

Optimization: Early Return​

Related Problems​

Takeaways​

Example(s)

Solution

Alternative Solution (Single Pass with Direction)

Complexity

Approach

Key Insights

Step-by-Step Example

Visual Representation

Edge Cases

Important Notes

Optimization: Early Return

Related Problems

Takeaways