洛谷 P2896 [USACO08FEB]Eating Together S

题目传送门

解题思路:

求一遍最长不下降和不上升子序列,然后总长度减去较大的长度.

AC代码:

#include<iostream>
#include<cstdio>
#include<algorithm>

using namespace std;

int n,a[30001],f[30001],dp[30001],len,tot;

int main() {
    scanf("%d",&n);
    for(int i = 1;i <= n; i++)
        scanf("%d",&a[i]);
    f[++len] = a[1];
    for(int i = 2;i <= n; i++) {
        if(a[i] >= f[len]) {
            f[++len] = a[i];
            continue;
        }
        int u = upper_bound(f+1,f+len+1,a[i]) - f;
        f[u] = a[i];
    }
    dp[++tot] = a[1];
    for(int i = 2;i <= n; i++) {
        if(a[i] <= dp[tot]) {
            dp[++tot] = a[i];
            continue;
        }
        int u = upper_bound(dp+1,dp+len+1,a[i],greater<int>()) - dp;
        dp[u] = a[i]; 
    }
    printf("%d",n - max(tot,len));
    return 0;
}