AC代码

#include <bits/stdc++.h>
using namespace std;
struct node {
	int left,right;
};
node tree[101];
void preorder(int root,int a){
	if(root==0){
		return;
	}
	if(a==1)
		cout<<root<<" ";
	preorder(tree[root].left,a);
	if(a==2)
		cout<<root<<" ";
	preorder(tree[root].right,a);
	if(a==3)
		cout<<root<<" ";
}
int main() {
	int n;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		int idx, left, right;
		cin >> idx >> left >> right;
		tree[idx].left = left;
		tree[idx].right = right;
	}
	int root = 1;
	preorder(root,1);
	cout << endl;
	preorder(root,2);
	cout << endl;
	preorder(root,3);
	return 0;
}