[LeetCode] 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 <= A.length <= 50000
-100000 <= A[i] <= 100000

Solution

class Solution {
    public boolean isMonotonic(int[] A) {
        if (A.length <= 2) return true;
        int i = 1;
        while (i < A.length && A[i] == A[i-1]) {
            i++;
        }
        if (i == A.length) return true;
        else if (A[i] > A[i-1]) return isInc(A, i);
        else return isDec(A, i);
    }
    private boolean isInc(int[] A, int start) {
        for (int i = start; i < A.length; i++) {
            if (A[i] < A[i-1]) return false;
        }
        return true;
    }
    private boolean isDec(int[] A, int start) {
        for (int i = start; i < A.length; i++) {
            if (A[i] > A[i-1]) return false;
        }
        return true;
    }
}

Two flag

class Solution {
    public boolean isMonotonic(int[] A) {
        int inc = 0, dec = 0;
        for (int i = 1; i < A.length; i++) {
            if (A[i] > A[i-1]) inc = 1;
            if (A[i] < A[i-1]) dec = 1;
        }
        if (inc == 1 && dec == 1) return false;
        return true;
    }
}

Or

class Solution {
    public boolean isMonotonic(int[] A) {
        boolean inc = true, dec = true;
        for (int i = 1; i < A.length; i++) {
            inc &= A[i] >= A[i-1];
            dec &= A[i] <= A[i-1];
        }
        return inc || dec;
    }
}