看到题目的第一眼就感到十分熟悉。。。后来才发现是半年前心血来潮（一时脑残）打CF现场赛时的题目。。。当时果真的太弱了。。这种题目都不会做。。。
其实是很简单的记忆化搜索。。枚举子树的大小考虑一下限制什么的乱搞（口胡）一下就好了。。
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int mod = 1e9+7;
int a[510],n,vis[510][510],f[510][510];
int read(){
int x = 0,f = 1,c = getchar();
while (c<'0' || c>'9'){if (c=='-') f = -1;c = getchar();}
while (c>='0' && c<='9') x = x*10+c-'0',c = getchar();
return x*f;
}
int dfs(int l,int r){
if (l>r) return f[l][r] = 1;
if (f[l][r]) return f[l][r];
int res = dfs(l+1,r),pos = r;
for (int i=l;i<r;i++)
if (a[l]<a[i+1]) (res+=1ll*dfs(l+1,i)*dfs(i+1,r)%mod)%=mod;
return f[l][r] = res%mod;
}
int main(){
// freopen("tree.in","r",stdin);
// freopen("tree.out","w",stdout);
n = read();
for (int i=1;i<=n;i++) a[i] = read();
printf("%d",dfs(2,n));
}
2022年5月23日 20:33
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