【題目】

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

【題意】

判斷是否是單調數組。

【解答】

用兩個bool變量分別記錄是否出現neg差值和pos差值，單調數組的話，兩個不可能同時為true。

時間O(N) 空間O(1)

class Solution {
public:
    bool isMonotonic(vector<int>& A) {
        int len = A.size();
        bool isNeg = false;
        bool isPos = false;
        for(int i=0; i+1<len; i++){
            int diff = A[i+1] - A[i];
            if(diff < 0){
                isNeg 
 
= true;
                if(isPos) return false;
            }
            if(diff > 0){
                isPos = true;
                if(isNeg) return false;
            }
        }
        return true;
    }
};

896. Monotonic Array