• 链接：https://www.acwing.com/problem/content/1016/

算法

目标：所有形状先上升后下降的子序列的长度最大值
两次LIS求解

代码

#include <iostream>
using namespace std;

const int N = 1010;
int n;
int a[N], f[N], g[N];

int main() {
    cin >> n;
    for (int i = 1; i <= n; ++i) cin >> a[i];
    
    for (int i = 1; i <= n; ++i) {
        f[i] = 1;
        for (int j = 1; j < i; ++j) {
            if (a[i] > a[j]) f[i] = max(f[i], f[j] + 1);
        }
    }
    for (int i = n; i; --i) {
        g[i] = 1;
        for (int j = n; j > i; --j) {
            if (a[i] > a[j]) g[i] = max(g[i], g[j] + 1);
        }
    }
    int ans = 0;
    for (int i = 1; i <= n; ++i) ans = max(ans, f[i] + g[i] - 1);
    cout << ans << endl;
    return 0;
}

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