896. Monotonic Array #

Problem #

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

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

Return true if and only if the given array A is monotonic.

Example 1:

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

Example 2:

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

Example 3:

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

Example 4:

Input: [1,2,4,5]
Output: true

Example 5:

Input: [1,1,1]
Output: true

Note:

1. 1 <= A.length <= 50000
2. -100000 <= A[i] <= 100000

Problem Summary #

If an array is monotone increasing or monotone decreasing, then it is monotonic. If for all i <= j, A[i] <= A[j], then array A is monotone increasing. If for all i <= j, A[i] >= A[j], then array A is monotone decreasing. Return true when the given array A is a monotonic array; otherwise return false.

Solution Approach #

• Determine whether the given array is monotonic (monotone increasing or monotone decreasing).
• Simple problem; just loop and check according to the problem statement.

Code #

package leetcode

func isMonotonic(A []int) bool {
	if len(A) <= 1 {
		return true
	}
	if A[0] < A[1] {
		return inc(A[1:])
	}
	if A[0] > A[1] {
		return dec(A[1:])
	}
	return inc(A[1:]) || dec(A[1:])
}

func inc(A []int) bool {
	for i := 0; i < len(A)-1; i++ {
		if A[i] > A[i+1] {
			return false
		}
	}
	return true
}

func dec(A []int) bool {
	for i := 0; i < len(A)-1; i++ {
		if A[i] < A[i+1] {
			return false
		}
	}
	return true
}