后序+中序，先序+中序，建立二叉树

建立二叉树（链式存储）

一、给定后序和中序数列，建立二叉树

#include<iostream>
#include<vector>
#include<unordered_map> 
using namespace std;
typedef struct node* BT; 
struct node{
    int data;
    BT left=NULL,right=NULL;
};

vector<int> Post,In;//存储后序遍历和中序遍历的元素
unordered_map<int,int> mp;//映射中序数组和下标的关系
 
BT BuildTree(int PostIndex,int InLeft,int InRight);
void Postorder(BT t);//后序遍历输出，用于检测正确性 

int main(void)
{
    int N;
    scanf("%d",&N);
    Post.resize(N+1);
    In.resize(N+1);
    for(int i=1;i<=N;i++)
        scanf("%d",&Post[i]);
    for(int i=1;i<=N;i++){
        scanf("%d",&In[i]);
        mp[In[i]]=i;
    }
    BT tree=NULL;
    tree=BuildTree(N,1,N);
    Postorder(tree);
    return 0;
 } 

void Postorder(BT t)
{
    if(t!=NULL){
        Postorder(t->left);
        Postorder(t->right);
        printf("%d ",t->data);
    }
}

BT BuildTree(int PostIndex,int InLeft,int InRight)
{
    if(InLeft>InRight) return NULL;
    BT t=new node();
    t->data=Post[PostIndex];
    t->right=BuildTree(PostIndex-1,mp[Post[PostIndex]]+1,InRight);
    int next=PostIndex-InRight+mp[Post[PostIndex]]-1;//下一个元素在Post中的位置即当前节点的PostIndex减去它的右子树的元素个数再-1 
    t->left=BuildTree(next,InLeft,mp[Post[PostIndex]]-1);
    return t;
}

二、给定先序和中序数列，建立二叉树

#include<iostream>
#include<vector>
#include<unordered_map>
using namespace std;
typedef struct node *BT;
struct node{
    int data;
    BT left=NULL,right=NULL;
};

vector<int> Pre,In;
unordered_map<int,int> mp;

void Preorder(BT t);
BT BuildTree(int PreIndex,int InLeft,int InRight);

int main(void)
{
    int N;
    scanf("%d",&N);
    Pre.resize(N+1);
    In.resize(N+1);
    for(int i=1;i<=N;i++)
        scanf("%d",&Pre[i]);
    for(int i=1;i<=N;i++){
        scanf("%d",&In[i]);
        mp[In[i]]=i;
    }
    BT tree=BuildTree(1,1,N);
    Preorder(tree);
    return 0;
}

void Preorder(BT t)
{
    if(t!=NULL){
        printf("%d ",t->data);
        Preorder(t->left);
        Preorder(t->right);
    }
}

BT BuildTree(int PreIndex,int InLeft,int InRight)
{
    if(InLeft>InRight) return NULL;
    BT t=new node();
    t->data=Pre[PreIndex];
    t->left=BuildTree(PreIndex+1,InLeft,mp[Pre[PreIndex]]-1);
    int next=PreIndex+mp[Pre[PreIndex]]-InLeft+1;
    t->right=BuildTree(next,mp[Pre[PreIndex]]+1,InRight);
    return t;
}