PAT A1043 Is It a Binary Search Tree [二叉查找树及遍历]

题目描述

链接
给定一个整数键值序列，现请你编写程序，判断这是否是对一棵二叉搜索树或其镜像进行前序遍历的结果

分析

• 注意每个地方二叉查找树的定义不同，看清题目
• 写insert函数的时候，不要忘记左右子树为NULL
• 镜像二叉查找树实际就是遍历的时候，把左右次序更换下就好了！

#include<bits/stdc++.h>
using namespace std;

const int maxn = 1005;
struct node{
    int data;
    node *lchild;
    node *rchild;
}nodes[maxn];

int a[maxn], n;

void insert(node *&root, int x){
    if(root == NULL){
        root = new node;
        root->data = x;
        root->lchild = NULL; //不要忘记!
        root->rchild = NULL; //不要忘记!
        return;
    }
    if(x < root->data) insert(root->lchild, x); //题目定义不同
    else insert(root->rchild, x);
}

node *create(){
    node *root = NULL;
    for(int i=0;i<n;i++){
        insert(root, a[i]);
    }
    return root;
}

vector<int> b,c,d;
void preorder(node *root){
    if(root==NULL) return;
    b.push_back(root->data);
    preorder(root->lchild);
    preorder(root->rchild);
}

void preorder2(node *root){
    if(root==NULL) return;
    c.push_back(root->data);
    preorder2(root->rchild);
    preorder2(root->lchild);
}

void postorder(node *root){
    if(root==NULL) return;
    postorder(root->lchild);
    postorder(root->rchild);
    d.push_back(root->data);
}

void postorder2(node *root){
    if(root==NULL) return;
    postorder2(root->rchild);
    postorder2(root->lchild);
    d.push_back(root->data);
}

int main(){
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>a[i];
    }
    node *root = create();
    preorder(root);
    preorder2(root);
    bool flag1 = true, flag2 = true;
    for(int i=0;i<n;i++){
        if(a[i] != b[i]) {flag1 = false; break;}
    }
    for(int i=0;i<n;i++){
        if(a[i] != c[i]) {flag2 = false; break;}
    }
    if(flag1){
        postorder(root);
        cout<<"YES"<<endl;
        for(int i=0;i<n;i++){
            if(i==0) cout<<d[i];
            else cout<<" "<<d[i];
        }
        cout<<endl;
    }else if(flag2){
        postorder2(root);
        cout<<"YES"<<endl;
        for(int i=0;i<n;i++){
            if(i==0) cout<<d[i];
            else cout<<" "<<d[i];
        }
        cout<<endl;
    }else{
        cout<<"NO"<<endl;
    }

}