9.19
完成了二叉树先中后递归和非递归遍历，以及层序遍历

#include<iostream>
#include<queue>
#include<stack>
using namespace std;
struct TreeNode{
    char val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x),left(left), right(right) {}
};
void preorder(TreeNode* root){
    if(root==NULL) return;
    cout<<root->val<<' ';
    preorder(root->left);
    preorder(root->right);
}
void inorder(TreeNode* root){
    if(root==NULL) return;
    inorder(root->left);
    cout<<root->val<<' ';
    inorder(root->right);
}
void postorder(TreeNode* root){
    if(root==NULL) return;
    postorder(root->left);
    postorder(root->right);
    cout<<root->val<<' ';
}
void levelorder(TreeNode* root){
    queue<TreeNode*> q;
    q.push(root);
    while(q.size()){
        TreeNode* node=q.front();
        q.pop();
        cout<<node->val<<' ';
        if(node->left) q.push(node->left);
        if(node->right) q.push(node->right);
    }
}
void preorder2(TreeNode* root){
    stack<TreeNode*> s;
    s.push(root);
    while(s.size()){
        TreeNode* node=s.top();
        s.pop();
        cout<<node->val<<' ';
        if(node->right) s.push(node->right);
        if(node->left) s.push(node->left);
    }

}
void inorder2(TreeNode* root){
    stack<TreeNode*> s;
    TreeNode* curr=root;
    while(curr||s.size()){
        while(curr){
            s.push(curr);
            curr=curr->left;
        }
        curr=s.top();
        s.pop();
        cout<<curr->val<<' ';
        curr=curr->right;
    }
}
void postorder2(TreeNode* root){
    stack<TreeNode*> s;
    TreeNode* curr=root;
    TreeNode* prev=NULL;
    while(curr||s.size()){
        while(curr){
            s.push(curr);
            curr=curr->left;
        }
        curr=s.top();
        if(curr->right==NULL||curr->right==prev){
            s.pop();
            cout<<curr->val<<' ';
            prev=curr;
            curr=NULL;
        }else curr=curr->right;
    }
}
int main()
{
    TreeNode *root = new TreeNode('A',
                                  new TreeNode('B',
                                               new TreeNode('D'),
                                               new TreeNode('E')),
                                  new TreeNode('C'));
    // preorder(root);
    // cout<<endl;
    // inorder(root);
    // cout<<endl;
    postorder(root);
    cout<<endl;
    // levelorder(root);
    // cout<<endl;
    // preorder2(root);
    // cout<<endl;
    // inorder2(root);
    // cout<<endl;
    postorder2(root);
    return 0;
}