思路： 贪心，

每次删除最上面的边。。

#include<utility>
#include<iostream>
#include<vector>
#include<cstring>
#include<deque>
#include<cstdio>
#include<algorithm>
using namespace std;

const int M = 200008;

deque <int > q;
vector <int> vi;
int first_edge[M],next_edge[M],to[M],ty[M],sum;

void dfs(int u){
    if(sum < 0){
        sum += 2;
        ty[q.front()] *= -1;
        vi.push_back(q.front());
        q.pop_front();
    }
    for(int i = first_edge[u] ; i!=-1 ; i = next_edge[i]){
        sum += ty[i];
        if(ty[i] == -1)q.push_back(i);
        dfs(to[i]);
        if(ty[i] == -1){
            q.pop_back();
        }
        sum -= ty[i];
    }
}

int main(){
    freopen("input.txt","r",stdin);
    int n,m,u,v;
    sum = 0;
    char buf[10];
    memset(first_edge,-1,sizeof(first_edge));
    scanf("%d",&n);
    for(int i = 0; i < n-1; i++){
       scanf("%d%d%s",&u,&v,buf);
       next_edge[i]  = first_edge[v];
       first_edge[v] = i;
       ty[i] = *buf == 'p' ? 1 : -1;
      // cout <<ty[i] <<endl;
       to[i] = u;
       if(*buf == 'a')scanf("%s",buf);
    }
    dfs(1);
    printf("%d\n",(int)vi.size());
    for(int i = 0; i < vi.size(); i++){
        printf("%d%c",vi[i]+1, i==vi.size() - 1 ? '\n': ' ');
    }
}

SGU 321 知道了双端队列，