二叉树非递归遍历

介绍二叉树的非递归算法（利用栈和父节点），实现相比较来说较为简单，代码更加简洁，逻辑清晰

一、先序遍历

<1>栈模拟实现

void PreorderTraversal(BinaryTree T)
{
    stack S;

    while (T != nullptr || !S.empty())
    {
        if (T != nullptr)
        {
            Visit(T);
            S.push(T);
            T = T->left;
        }
        else
        {
            T = S.pop();
            T = T->right;
        }
    }
}

<2>父节点实现

void PreOrderTransval(BinaryTree T)
{
    while (T != nullptr)
    {
        Visit(T);
        if (T->left != nullptr)
            T = T->left;
        else
        if (T->right != nullptr)
            T = T->right;
        else
        {
            BinaryTree child;
            while (1)
            {
                child = T;
                T = T->parent;
                if (T == nullptr) break;
                if (T->right != nullptr && T->left == child)
                {
                    T = T->right;
                    break;
                }
            }
        }
    }
}

二、中序遍历

<1>栈模拟实现

void InorderTraversal(BinaryTree T)
{
    stack S;
    while (T != nullptr || !S.empty())
    {
        if (T != nullptr)
        {
            S.push(T);
            T = T->left;
        }
        else
        {
            T = S.pop( );
            Visit(T);
            T = T->right;
        }
    }
}

<2>父节点实现

void InorderTraversal(BinaryTree T)
{
    int mark = 0;
    while (T != nullptr)
    {
        if (mark == 0)
        {
            if (T->left != nullptr)
                T = T->left;
            else
                mark = 1;
        }
        else
        {
            BinaryTree child;

            Visit(T);
            if (T->right != nullptr)
            {
                T = T->right;
                mark = 1;
            }
            else
            {
                while(1)
                {
                    child = T;
                    T = T->parent;
                    if (T == nullptr) break;
                    if (T->left == child)
                    {
                        mark = 1;
                        break;
                    }
                }
            }
        }
    }
}

三、后序遍历

<1>栈模拟

typedef struct
{
    BinaryTree node = nullptr;
    int mark = 0;
}Node;

void PostOrder(BinaryTree bt)
{
    stack<Node> s;
    Node tmpCell;

    while (bt || !s.empty())
    {
        if (bt)
        {
            tmpCell.node = bt;
            tmpCell.mark = 0;
            s.push(tmpCell);
            bt = bt->left;
        }
        else
        {
            tmpCell = s.top( );
            s.pop( );
            if (tmpCell.mark == 0)
            {
                tmpCell.mark = 1;
                s.push(tmpCell);
                bt = bt->right;
            }
            else
            {
                Visit(tmpCell.node);
                bt = nullptr;
            }
        }
    }
}