寻找不变值求可变值(排名)

题目链接

https://codeforces.com/problemset/problem/1153/D

题目大意

题目思路

定义dp[u]为u节点在其子树中的排名为多少？ ！太妙了！

题目代码

#include<iostream>
#include<cstring>
#include<algorithm>
#include<vector>
#define ll long long
const int inf = 0x7fffffff;
const int N = 3e5 + 5;
using namespace std;
int n,k;
int op[N],fa[N],s[N],dp[N];
vector<vector<int>>g(N);
void dfs1(int u,int fa)
{
	for(int v:g[u])
	{
		if(v == fa) continue;
		dfs1(v,u);
		s[u] += s[v];
	}
	// 如果是叶子节点 
	if(g[u].size() == 1 && u != -1){
		s[u] = 1;
		dp[u] = 1;
	}
	return; 
}
void dfs2(int u,int fa)
{
	if(g[u].size() == 1 && u != -1) return;
	if(op[u] == 1){
		dp[u] = inf;
		for(int v:g[u])
		{
			if(v == fa) continue;
			dfs2(v,u);
			dp[u] = min(dp[u],dp[v]);
		}
	}else{
		for(int v:g[u])
		{
			if(v == fa) continue;
			dfs2(v,u);
			dp[u] += dp[v];
		}
	}
}
int main()
{
	cin >> n;
	for(int i = 1;i <= n;++i) cin >> op[i];
	for(int i = 2;i <= n;++i){
		int x;cin >> x;
		g[x].push_back(i);
		g[i].push_back(x);
	}
	dfs1(1,0);dfs2(1,0);
	cout << s[1] - dp[1] + 1 << '\n';
	return 0;
}