【Leetcode】1395. Count Number of Teams

题目地址：

https://leetcode.com/problems/count-number-of-teams/

给定一个长$n$数组$A$，题目保证无重复数字，问其有多少个三元组$0\le i<j<k<n$使得$A[i]<A[j]<A[k]$或者$A[i]>A[j]>A[k]$。

枚举中间数字，然后累加一下左边比它小的个数乘以右边比它大的个数，再累加左边比它大的个数乘以右边比它小的个数。代码如下：

public class Solution {
     
    public int numTeams(int[] rating) {
     
        int res = 0;
        for (int i = 1; i < rating.length - 1; i++) {
     
            int leftGreater = 0, rightGreater = 0, leftSmaller = 0, rightSmaller = 0;
            for (int j = 0; j < i; j++) {
     
                if (rating[j] > rating[i]) {
     
                    leftGreater++;
                } else {
     
                    leftSmaller++;
                }
            }
            
            for (int j = i + 1; j < rating.length; j++) {
     
                if (rating[j] > rating[i]) {
     
                    rightGreater++;
                } else {
     
                    rightSmaller++;
                }
            }
            
            res += leftSmaller * rightGreater;
            res += leftGreater * rightSmaller;
        }
        
        return res;
    }
}

时间复杂度$O(n^2)$，空间$O(1)$。