树的先序、中序、后序和层次遍历的C++递归实现

树的遍历及遍历规则：
先序遍历：根左右
(即先访问根节点,在访问左儿子，最后访问右儿子)
中序遍历：左根右
后序遍历：左右根
层次遍历：按树的深度从小到大一层一层遍历

C++实现(递归)：

#include
using namespace std;
const int N=1e6+5;
int dep[N];
int treedep=0;
vector v[30];
struct node{
	int v,Next;	
}e[N*2];
int first[N];
int n,u1,v1,k=0;
void add(int ui,int vi){
	++k; e[k].v=vi; e[k].Next=first[ui]; first[ui]=k;
}
void xianxu(int ui,int father){
	printf("%d ",ui);
	for(int i=first[ui];i;i=e[i].Next)
	{
		int vi=e[i].v; if(vi==father) continue;
		xianxu(vi,ui);
	}
}
void zhongxu(int ui,int father){
	int root=ui,lchild=0,rchild=0;
	for(int i=first[ui];i;i=e[i].Next)
	{
		int vi=e[i].v; if(vi==father) continue;
		if(lchild) rchild=vi;	
		else lchild=vi; 
	}
	if(lchild) zhongxu(lchild,root);
	printf("%d ",root);
	if(rchild) zhongxu(rchild,root);
}
void houxu(int ui,int father){
	int root=ui,lchild=0,rchild=0;
	for(int i=first[ui];i;i=e[i].Next)
	{
		int vi=e[i].v; if(vi==father) continue;
		if(lchild) rchild=vi;
		else lchild=vi;
	}
	if(lchild) houxu(lchild,root);
	if(rchild) houxu(rchild,root);
	printf("%d ",root);
}
void cengci(int ui,int father){
	dep[ui]=dep[father]+1;
	treedep=max(treedep,dep[ui]);
	v[dep[ui]].push_back(ui);
	int lchild=0,rchild=0;
	for(int i=first[ui];i;i=e[i].Next)
	{
		int vi=e[i].v; if(vi==father) continue;
		cengci(vi,ui);
	}
}
int main(){
	//freopen("tree.in","r",stdin);
	//freopen("tree.out","w",stdout);
	memset(first,0,sizeof(first));
	memset(dep,0,sizeof(dep));
	scanf("%d",&n);
	for(int i=1;i
		scanf("%d %d",&u1,&v1); add(u1,v1); add(v1,u1);
	}
	printf("pretravel:n");
	xianxu(1,0);
	printf("nmidtravel:n");
	zhongxu(1,0);
	printf("nsufftravel:n");
	houxu(1,0);
	printf("ndeptravel:n");
	cengci(1,0);
	for(int i=1;i<=treedep;++i)
		for(int j=0;j