洛谷官方好像没有完整的题解
所以放一下我的AC代码.

#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 10;
int n;
class node
{
    public:
        int key;//当前数据记录的数值 
        node* left;//左节点记录的数值 
        node* right;//右节点记录的数值 
        node(int data) : key(data),left(nullptr),right(nullptr){}
};
using bnode = node*;
vector<bnode> Tree(N,nullptr);
void buildTree(int n,vector<pair<int,int>> tree_node)
{ 
    for(int i = 1;  i <= n ; i++){
        Tree[i] = new node(i);
    } 
    for(int i = 1 ; i <= n ; i++)
    {
        int l = tree_node[i-1].first;
        int r = tree_node[i-1].second;
        if(l != 0)Tree[i]->left = Tree[l];
        if(r != 0) Tree[i]->right = Tree[r];
    }
}
void preIntraversal(node* root)
{
    if(root == nullptr) return;
    cout << root->key << " ";
    preIntraversal(root->left);
    preIntraversal(root->right); 
}
void inordTraversal(bnode root)
{
    if(root == nullptr) return;
    inordTraversal(root->left);
    cout << root->key << " ";
    inordTraversal(root->right);
}
void posTraversal(bnode root)
{
    if(root == nullptr) return;
    posTraversal(root->left);
    posTraversal(root->right);
    cout << root->key << " ";
}
int main()
{
    cin >> n;
    vector<pair<int,int>> tree_node;
    int a,b;
    for(int i = 1 ; i <= n ; i++)
    {
        cin >> a >> b;
        tree_node.push_back({a,b});
    }
    buildTree(n,tree_node);
    bnode root = Tree[1];
    preIntraversal(root);cout << endl;
    inordTraversal(root);cout << endl;
    posTraversal(root);
    return 0;
}