数组存储
#include <iostream>
#include <cstdio>
using namespace std;
const int N = 1010;
int n;
int a[N];
int main(){
    scanf("%d",&n);
    for(int i = 0;i < n;++i)    scanf("%d",&a[i]);
    int ans = 0;
    for(int i = 1;i < n - 1;++i){
        if((a[i - 1] < a[i] && a[i] > a[i + 1]) || (a[i - 1] > a[i] && a[i] < a[i + 1]))    ans++;

    }
    printf("%d",ans);
    return 0;
}

判断单调性
#include <iostream>
#include <algorithm>
#include <cstdio>
using namespace std;
int n,ans = 0;
int main(){
    cin >> n;
    int left,mid,right;
    if(cin >> left >> mid){
        for(int i = 3;i <= n;++i){
            cin >> right;
            if(left < mid && mid > right)   ans++;
            else if(left > mid && mid < right)  ans++;
            left = mid;
            mid = right;
        }
    }
    cout << ans << endl;
    return 0;
}

单调区间与区间之间比较，如果不同则为折点
#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
int n,ans = 0;
int first,second,prearrow,arrow;
int main(){
    cin >> n;
    cin >> first;
    if(cin >> second){
        prearrow = (second - first > 0) ? 1 : 0;
        first = second;
        for(int i = 3;i <= n;++i){
            cin >> second;
            arrow = (second - first > 0) ? 1 : 0;
            if(prearrow != arrow)   ans++;
            prearrow = arrow;
            first = second;
        }
    }
    cout << ans << endl;
    return 0;
}