用差分法

#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;

const int N = 500000;
const int M = 10000;
int a[N + 2 ]= { 0 };
int d[M + 1] = { 0 };

int main()
{
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    a[0] = a[n + 1] = 0;

    n = unique(a, a + n + 2) - a - 1;//去重

    memset(d, 0, sizeof d);
    for (int i = 1; i < n; i++)
        if (a[i - 1] < a[i] && a[i] > a[i + 1])
        {
            d[a[i]]++;
        }
        else if (a[i - 1] > a[i] && a[i] < a[i + 1])
        {
            d[a[i]]--;
        }

    int ans = 0, sum = 0;   // 差分前缀和即为答案
    for (int i = M; i >= 1; i--)
    {
        sum += d[i];
        ans = max(ans, sum);
    }


    cout << ans << endl;

    return 0;
}