896. Monotonic Array

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

方法1:

class Solution1 {
public:
    bool isMonotonic(vector<int>& A) {
        if (A.size() <= 1) return true;
        if (A[0] <= A.back()) {
            for (int i = 0; i < A.size() - 1; i++) {
                if (A[i] > A[i + 1]) return false;
            }
        }
        else {
            for (int i = 0; i < A.size() - 1; i++) {
                if (A[i] < A[i + 1]) return false;
            }
        }
        return true;
    }
};

方法2: one pass

不用判空。

class Solution {
public:
    bool isMonotonic(vector<int>& A) {
        bool inc = true;
        bool dec = true;
        
        for (int i = 0; i < A.size() - 1; i++) {
            inc &= (A[i] <= A[i + 1]);
            dec &= (A[i] >= A[i + 1]);
        }
        return inc || dec;
    }
};