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:

  1. Input: [1,2,2,3]
  2. 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

解题

两种 CPP 的方法耗时几乎相等，方法 1 依照错误先行的逻辑处理，在特定情况下会更快，方法 2 为参考用例

CPP 版本

class Solution {
public:
    bool isMonotonic(vector<int>& A) {
        if (A.size() < 2) {
            return true;
        }
        bool increase = false;
        bool decrease = false;
        for (int i = 1; i < A.size(); i++) {
            if (A[i] > A[i - 1]) {
                increase = true;
            }
            if (A[i] < A[i - 1]) {
                decrease = true;
            }
            if (increase && decrease) {
                return false;
            }
        }
        return true;
    }
};

JS 版本

/**
 * @param {number[]} A
 * @return {boolean}
 */
var isMonotonic = function(A) {
    if (A.length < 2) {
        return true;
    }
    let increase = false;
    let decrease = false;
    for (let i = 1; i < A.length; i++) {
        if (A[i] > A[i - 1]) {
            increase = true;
        }
        if (A[i] < A[i - 1]) {
            decrease = true;
        }
        if (increase && decrease) {
            return false;
        }
    }
    return true;
};

全局“与”方案

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

全局“与”方案+错误先行

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