算法-二叉树的前中后遍历stack版

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二叉树的前中后遍历stack版
    /// <summary>    /// TraversalTreeUsingStack    /// </summary>    public class TraversalTreeUsingStack    {        private Stack<TreeNode> _stack = new Stack<TreeNode>();        //前序遍历        public void PreOrder(TreeNode root)        {            _stack.Clear();            if (root == null)                return;            TreeNode tmp = root;            _stack.Push(tmp);//先访问根节点            while (_stack.Count > 0)            {                if (tmp == null)//左子树为空                                 tmp = _stack.Peek(); //弹出右子树                visit(_stack.Peek());                _stack.Pop();                if (tmp.right != null)                    _stack.Push(tmp.right);                if (tmp.left != null)                    _stack.Push(tmp.left);                tmp = tmp.left;//迭代等式            }        }        //中序遍历        //先遍历左孩子，再访问根节点，最后遍历右孩子        public void MidOrder(TreeNode root)        {            _stack.Clear();            if (root == null) return;            _stack.Push(root);            while (_stack.Count > 0)            {                TreeNode tmp = _stack.Peek();                while (tmp != null) //遍历左子树的最孩子，直到为null                {                    _stack.Push(tmp.left);                    tmp = tmp.left;                }                //可能1 左孩子为null时退出上层循环，但是此时栈顶为null                //可能2 栈顶为null，while直接退出                _stack.Pop(); //一定弹出null                       if (_stack.Count == 0) return;                visit(_stack.Peek()); //访问节点                    var tmp2 = _stack.Pop();                _stack.Push(tmp2.right);//如果右孩子为null，则null入栈。            }        }        //后序遍历        public static IList<T> PostorderTraversal<T>(TreeNode<T> root)        {            IList<T> rtn = new List<T>();            var s = new Stack<TreeNode<T>>();            if (root == null) return rtn;            s.Push(root);            TreeNode<T> cur = root;            TreeNode<T> r = null; //标记访问过的右子树            while (s.Count > 0)            {                if (cur != null && cur.left != null)                {                    s.Push(cur.left);                    cur = cur.left;                }                else                {                    cur = s.Peek();                    if (cur.right != null && cur.right != r)                    {                        cur = cur.right;                        s.Push(cur);                    }                    else                    {                        rtn.Add(s.Pop().val);                        r = cur;                        cur = null;                    }                }            }            return rtn;        }        private void visit(TreeNode node)        {            if (node != null)                Console.WriteLine(node.val);        }    }
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