LeetCode 896. Monotonic Array

原题链接在这里：https://leetcode.com/problems/monotonic-array/

题目：

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

题解：

Having a direction as d. If it is increasing, d = 1. If it is decreasing, d = -1.

When seeing a increasing, but d = -1, that means there is decreasing before, return false.

Vice Versa.

Time Complexity: O(n). n = A.length.

Space: O(1).

AC Java:

class Solution {
    public boolean isMonotonic(int[] A) {
        if(A == null || A.length == 0){
            return true;
        }
        
        int d = 0;
        for(int i = 1; i<A.length; i++){
            if(A[i] > A[i-1]){
                if(d < 0){
                    return false;
                }
                
                d = 1;
            }else if(A[i] < A[i-1]){
                if(d > 0){
                    return false;
                }
                
                d = -1;
            }
        }
        
        return true;
    }
}

类似Count Hills and Valleys in an Array.