二叉树的建立和前序，中序，后序，遍历（给定节点以及其之间的关系）

其中键值对的建立最为关键，不熟悉unorder_maphttps://blog.csdn.net/zou_albert/article/details/106983268?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522169528103016800211567279%2522%252C%2522scm%2522%253A%252220140713.130102334..%2522%257D&request_id=169528103016800211567279&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~all~top_positive~default-1-106983268-null-null.142^v94^insert_down28v1&utm_term=unordered_map&spm=1018.2226.3001.4187的同学，可以查看链接这篇文章， 

#include
#include
#include
using namespace std;
//节点结构体
struct treenode {
    char val;
    treenode* left;
    treenode* right;
    treenode(char val) :val(val), left(nullptr), right(nullptr) {}//此处{},和char val不能忘记了
};
 
//建立二叉树
treenode* buildtree(unordered_map<char, pair<char,char>>& nodemap, char rootvalue)//需要参数：键值对，根节点
{
    if (rootvalue == '0')
        return nullptr;
    treenode* root = new treenode(rootvalue);
    char leftchild = nodemap[rootvalue].first;
    char rightchild= nodemap[rootvalue].second;
    root->left = buildtree(nodemap, leftchild);
    root->right = buildtree(nodemap, rightchild);
    return root;
}
 
 
 
void pretree(treenode* root)
{
    if (root == nullptr)
    {
        return;
    }
    cout << root->val;
    pretree(root->left);
    pretree(root->right);
}
void midtree(treenode* root)
{
    if (root == nullptr)
    {
        return;
    }
    midtree(root->left);
    cout << root->val;
    midtree(root->right);
}
void posttree(treenode* root)
{
    if (root == nullptr)
    {
        return;
    }
    posttree(root->left);
    posttree(root->right);
    cout << root->val;
}
int main()
{
    int n;
    cin >> n;
    unordered_map<char, pair<char, char>> nodemap;//将键类型与值进行映射，建立的无序哈希表
    vector<char>index(n+1, '0');//这是节点的字母，n+1维，因为题目中F的下标是从1开始计算的，数组同样
    vectorint>>num(n + 1, vector<int>(2,0));//这是建立了n+1维，2列的数组进行存储
    for (int i = 1; i <= n; i++)
    {
        cin >> index[i] >> num[i][0] >> num[i][1];
    }
    //生成键值对的nodemap
    for (int i = 1; i <= n; i++)
    {
        nodemap[index[i]] = make_pair(index[num[i][0]], index[num[i][1]]);//此处最应该注意，我在敲的时候忘记了加上index索引
    }
  
    //建立二叉树
    treenode* root = buildtree(nodemap, index[1]);
    pretree(root);
    cout << endl;
    midtree(root);
    cout << endl;
    posttree(root);
    cout << endl;
    return 0;
}