Very thoughtful recursive and non-recursive implementation of pre-order traversal of binary trees

Spoiler: I accidentally saw a good binary tree preorder example. The non-recursive operation is worth a look.

Pre-order traversal of a binary tree
The following figure represents a binary tree. Pre-order traversal operation is performed on it, using two methods, recursive and non-recursive operations. .

The traversal result is: 1245367.

1. Recursive operation:

Thought: If the binary tree is empty, return. otherwise

1) Traverse the root node; 2) Traverse the left subtree in order; 3) Traverse the right subtree in order

Code:

void PreOrder(BiTree root)  
{  
    if(root==NULL)  
        return ;  
    printf("%c ", root->data); //输出数据  
    PreOrder(root->lchild); //递归调用，先序遍历左子树  
    PreOrder(root->rchild); //递归调用，先序遍历右子树  
}  

2. Non-recursive operations

Idea: Non-recursive pre-order traversal of a binary tree. Pre-order traversal idea: Put the root into the stack first. As long as the stack is not empty, you can pop it up. Every time you pop up a node, remember to push its left and right nodes into the stack. , remember that the right subtree is stacked first, which ensures that the right subtree is always below the left subtree in the stack.

Code:

void PreOrder_Nonrecursive(BiTree T)     //先序遍历的非递归    
{  
    if(!T) return ;    
    stack<BiTree> s;  
    s.push(T);  
    while(!s.empty())  
    {  
        BiTree temp = s.top();  
        cout<<temp->data<<" ";  
        s.pop();  
        if(temp->rchild)  
            s.push(temp->rchild);  
        if(temp->lchild)  
            s.push(temp->lchild);  
    }  
}  

or:

void PreOrder_Nonrecursive(BiTree T)     //先序遍历的非递归  
{  
    if(!T) return ;  
    stack<BiTree> s;  
    while(T)          // 左子树上的节点全部压入到栈中  
    {  
        s.push(T);  
        cout<<T->data<<"  ";  
        T = T->lchild;  
    }        
    while(!s.empty())  
    {          
        BiTree temp = s.top()->rchild;  // 栈顶元素的右子树  
        s.pop();                        // 弹出栈顶元素  
        while(temp)          // 栈顶元素存在右子树，则对右子树同样遍历到最下方  
        {  
            cout<<temp->data<<"  ";  
            s.push(temp);  
            temp = temp->lchild;  
        }  
    }  
}