#include <bits/stdc++.h>

using namespace std;

typedef struct node{
	int data;
	struct node *lchild,*rchild;
	node(int n){
		data=n;
		lchild=NULL;
		rchild=NULL;
	}
}node,*Tree;

void createTree(Tree &T,Tree parent,int num){
	if(T==NULL){
		T=new node(num);
		if(parent==NULL) cout<<"-1"<<endl;
		else cout<<parent->data<<endl;
	}else if(num>T->data) createTree(T->rchild,T,num);
	else createTree(T->lchild,T,num);
}

int main(){
	int n,x;
	while(scanf("%d",&n)!=EOF){
		Tree T=NULL;
		while(n--){
			scanf("%d",&x);
			createTree(T,NULL,x);
		}
	}
}

二叉树模板

1，建树方式。层次输入建树，二叉搜索建树

2，输出方式。前序，中序，后序，层次