AcWing 187. 导弹防御系统（dfs）

题意

1. 直接 dfs，dp 不好 dp

代码

#include <bits/stdc++.h>
using namespace std;
#define db  double
#define ll  long long
#define Pir pair<int, int>
#define fi  first
#define se  second
#define pb  push_back
#define m_p make_pair
#define inf 0x3f3f3f3f
#define INF 0x3f3f3f3f3f3f3f3f
/*==========ACMer===========*/
const int N = 55;
int n, a[N], up[N], dn[N];
int ans = inf;

void dfs(int p, int su, int sd)
{
    
    
    if (su + sd >= ans) return;
    if (p == n)
    {
    
    
        ans = min(ans, su + sd);
        return;
    }


    int i;
    for (i = 1; i <= su; i ++) {
    
    
        if (a[p] > up[i]) break;
    }
    int tmp = up[i];
    up[i] = a[p];
    dfs(p + 1, max(su, i), sd);
    up[i] = tmp;

    for (i = 1; i <= sd; i ++) {
    
    
        if (a[p] < dn[i]) break;
    }
    tmp = dn[i];
    dn[i] = a[p];
    dfs(p + 1, su, max(sd, i));
    dn[i] = tmp;
}


int main()
{
    
    
    while (scanf("%d", &n) && n)
    {
    
    
        ans = inf;
        for (int i = 0; i < n; i ++) scanf("%d", &a[i]);
        dfs(0, 0, 0);
        printf("%d\n", ans);
    }

    return 0;
}