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;
}