[每日 LeetCode] 896. Monotonic Array

Description:

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

思路：本题要求判断数组是否是单调数组，即数组要么递增要么递减。想到一种方法，遍历数组，判断相邻两个差值的乘积是否小于 0，若小于 0 则返回 false，否则返回 true。

C++ 代码

class Solution {
public:
    bool isMonotonic(vector<int>& A) {
        if(A.size()<=1)
            return true;
        int last=0;
        for(int i=1;i<A.size();i++)
        {
            int diff=A[i]-A[i-1];
            if (last*diff<0)
                return false;
            if(diff!=0)
                last=diff;
        }
        return true;   
    }
};

运行时间：92ms

运行内存：13.9M