二叉树遍历

循环：利用叶子结点右指针为空的特点，给叶子结点设置其直接后继，输出完孩子结点后，再返回其直接后继；

void midPrint_m(TreeNode *root)
{
    TreeNode *cur = root;
    TreeNode *pre = NULL;
    while(cur != NULL)
    {
        if(cur->left == NULL)
        {//左孩子为空，则直接输出
            cout<<cur->var<<" ";
            cur = cur->right;
        }
        else
        {
            pre = cur->left;
            while( pre->right != NULL && pre->right != cur )
            {//找到cur的直接前驱
                pre = pre->right;
            }
            if(pre->right == NULL)
            {//设置后继结点
                pre->right = cur;
                cur = cur->left;
            }
            else
            {
                pre->right = NULL;//重新设置为空
                cout<<cur->var<<" ";
                cur = cur->right;
            }
        }
    }
}

void prePrint_m(TreeNode *root)
{//基本同于中遍历
    TreeNode *cur = root;
    TreeNode *pre = NULL;
    while(cur != NULL)
    {
        if(cur->left == NULL)
        {
            cout<<cur->var<<" ";
            cur = cur->right;
        }
        else
        {
            pre = cur->left;
            while( pre->right != NULL && pre->right != cur )
            {
                pre = pre->right;
            }
            if(pre->right == NULL)
            {
                pre->right = cur;
                cout<<cur->var<<" ";
                cur = cur->left;
            }
            else
            {
                pre->right = NULL;
                cur = cur->right;
            }
        }
    }
}
//this is the most difficult algorithm
void reverse_out(TreeNode *from,TreeNode *to)
{
    //first reverse from->to
    //reverse
    TreeNode *cur = from;
    TreeNode *post = cur->right;
    while(cur != to)
    {
        TreeNode *tmp = post->right;
        post->right = cur;
        cur = post;
        post = tmp;
    }
    //already reverse,output list
    TreeNode *traversal = cur;
    while( cur != from )
    {
        cout<<cur->var<<" ";
        cur = cur->right;
    }
    cout<<cur->var<<" ";
    //reverse original
    cur = to;
    post = cur->right;
    while(cur != from)
    {
        TreeNode *tmp = post->right;
        post->right = cur;
        cur = post;
        post = tmp;
    }
    //restore to's right
    to->right = NULL;
}
void postPrint_m(TreeNode *root)
{
    TreeNode *newroot = new TreeNode(0);
    newroot->left = root;
    newroot->right = NULL;
    TreeNode *cur = newroot;
    TreeNode *pre = NULL;
    while(cur != NULL)
    {
        if(cur->left == NULL)
        {
            cur = cur->right;
        }
        else
        {
            pre = cur->left;
            while(pre->right != NULL && pre->right != cur)
            {
                pre = pre->right;
            }
            if(pre->right == NULL)
            {
                pre->right = cur;
                cur = cur->left;
            }
            else
            {
                pre->right = NULL;
                reverse_out(cur->left,pre);
                cur = cur->right;
            }
        }
    }
}