二叉树系列四：二叉树的遍历（非递归）

在二叉树系列三中讲述了二叉树的前序遍历、中序遍历和后序遍历的递归实现，可以看到采用递归实现的代码非常简单，但是代码简单不代表实际运行过程中也能达到最简。下面将要介绍二叉树几种遍历的非递归实现实现。

（1）前序遍历 

template<class T>
void Tree<T>::PreOrderNonRec(TreeNode<T>* root)
{
    if(root != NULL)
    {
        stack<TreeNode<T>*> s;
        TreeNode<T>* curr = root;
        while(curr || !s.empty())
        {
            while(curr)
            {
                cout<<curr->data<<endl;
                s.push(curr);
                curr = curr->leftChild;
            }
            if(!s.empty())
            {
                curr = s.top();
                s.pop();
                curr = curr->rightChild;
            }
        }
    }
}

（2）中序遍历

template<class T>
void Tree<T>::MidOrderNonRec(TreeNode<T>* root)
{
    if(root != NULL)
    {
        TreeNode<T>* curr = root;
        stack<TreeNode<T>*> s;
        while(curr || !s.empty())
        {
            while(curr)
            {
                s.push(curr);
                curr = curr->leftChild;
            }
            if(!s.empty())
            {
                curr = s.top();
                s.pop();
                cout<<curr->data<<endl;
                curr = curr->rightChild;
            }
        }
    }
}

（3）后序遍历

template<class T>
struct StackItem
{
    TreeNode<T>* node;
    bool RCVisited;
};

template<class T>
void Tree<T>::PostOrderNonRec(TreeNode<T>* root)
{
    if(root != NULL)
    {
        TreeNode<T>* curr = root;
        stack<StackItem<T>> s;
        StackItem<T> item;
        while(curr || !s.empty())
        {
            while(curr)
            {
                item.node = curr;
                item.RCVisited = false;
                s.push(item);
                curr = curr->leftChild;
            }
            if(!s.empty())
            {
                item = s.top();
                s.pop();
                curr = item.node;
                if(item.RCVisited)
                {
                    cout<<curr->data<<endl;
                    curr = NULL;
                }
                else
                {
                    item.RCVisited = true;
                    s.push(item);
                    curr = curr->rightChild;
                }
            }
        }
    }
}