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 <= A.length <= 50000-100000 <= 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:
1 class Solution { 2 public boolean isMonotonic(int[] A) { 3 if(A == null || A.length == 0){ 4 return true; 5 } 6 7 int d = 0; 8 for(int i = 1; i<A.length; i++){ 9 if(A[i] > A[i-1]){ 10 if(d < 0){ 11 return false; 12 } 13 14 d = 1; 15 }else if(A[i] < A[i-1]){ 16 if(d > 0){ 17 return false; 18 } 19 20 d = -1; 21 } 22 } 23 24 return true; 25 } 26 }