二叉排序树(三序输出)

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
typedef struct Node{
    Node *l, *r;
    int v;
    Node(){l = NULL; r = NULL;}
}*tree, Node;
tree build(tree p, int v){
    if(p == NULL){
        p = new Node();
        p->v = v;
        return p;
    }
    if(v < p->v)
        p->l = build(p->l, v);
    else if(v > p->v)
        p->r = build(p->r, v);
    else
        return p;
    return p;
}
void PreOrder(tree p){
    if(p == NULL)return;
    printf("%d ", p->v);
    PreOrder(p->l);
    PreOrder(p->r);
}
void InOrder(tree p){
    if(p == NULL)return;
    InOrder(p->l);
    printf("%d ", p->v);
    InOrder(p->r);
}
void PostOrder(tree p){
    if(p == NULL)return;
    PostOrder(p->l);
    PostOrder(p->r);
    printf("%d ", p->v);
}
int main(){
    int N;
    while(~scanf("%d", &N)){
        tree p;
        p = NULL;
        int v;
        for(int i = 0; i < N; i++){
            scanf("%d", &v);
            p = build(p, v);
        }
        PreOrder(p);
        puts("");
        InOrder(p);
        puts("");
        PostOrder(p);
        puts("");
    }
    return 0;
}