896. Monotonic Array

Question:

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.

如果单调递增或单调递减，则数组是单调的。

如果对于所有i <j，a[i] <＝a[j]，数组A是单调递增的。如果对于所有i <j，a[i]＞a[j]，数组A是单调递减的。

当且仅当给定数组A为单调时返回真。

Example:

Input: [1,2,2,3]
Output: true
Input: [6,5,4,4]
Output: true
Input: [1,3,2]
Output: false
Input: [1,2,4,5]
Output: true
Input: [1,1,1]
Output: true

Solution:

/**
 * @param {number[]} A
 * @return {boolean}
 */
var isMonotonic = function(A) {
    var arr = [];
    for (var i = 0 ; i< A.length ; i ++) {
        arr.push(A[i+1] - A[i]);
    }
    return !arr.some(i=>i>0) || !arr.some(i=>i<0)
};

good solutions:

var isMonotonic = function(A) {
    let increase = true, decrease = true;
    for(let i = 0; i < A.length - 1; i++) {
        if(A[i] > A[i + 1]) increase = false;
        if(A[i] < A[i + 1]) decrease = false;
    }
    return increase || decrease;
};