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

Solutions (Rust)

1. Linear Scan

impl Solution {
    pub fn is_monotonic(a: Vec<i32>) -> bool {
        let mut flag = 0;

        for i in 1..a.len() {
            if a[i - 1] != a[i] {
                if flag * (a[i - 1] - a[i]) >= 0 {
                    flag = a[i - 1] - a[i];
                } else {
                    return false;
                }
            }
        }

        true
    }
}

2. Sort

impl Solution {
    pub fn is_monotonic(a: Vec<i32>) -> bool {
        let mut sort_a = a.clone();
        sort_a.sort_unstable();

        if sort_a == a {
            true
        } else {
            sort_a.reverse();
            sort_a == a
        }
    }
}