使用非递归函数和递归函数分别实现二叉数的前序丶中序丶后序遍历

最近在复习二叉树的算法，所以对二叉树的遍历分别做了用两中不同方式来实现二叉树遍历

首先是先序遍历

        /***********************先序遍历**************************/
        //先输出当前结点，再输出左结点，再输出右结点

        /// <summary>
        ///  使用非递归方式实现先序遍历
        /// </summary>
        /// <param name="head"></param>
        public static void PreOrderUnRecur(Node head)
        {
            //保证这个结点不为空
            if (head != null)
            {
                Stack<Node> stack = new Stack<Node>();
                //将结点压栈
                stack.Push(head);
                //循环将当前结点弹出并输出，然后压栈，先加右结点，再加左结点
                //这是为了保证下一个弹出的是左结点 例如弹出了A，然后压入C，再压入B
                //下个弹出的就会是B,然后依次循环直至没东西
                while (stack.Count != 0)
                {
                    head = stack.Pop();
                    Console.Write(head.value + " ");
                    if (head.right != null)
                    {
                        stack.Push(head.right);
                    }
                    if (head.left != null)
                    {
                        stack.Push(head.left);
                    }
                }
            }
        }

        /// <summary>
        /// 使用递归实现先序遍历
        /// </summary>
        /// <param name="head"></param>
        public static void PreOrderUnRecurs(Node head)
        {
            if (head == null) return;
            Console.Write(head.value + " ");
            PreOrderUnRecurs(head.left);
            PreOrderUnRecurs(head.right);
        }

接着是中序遍历

        /***********************中序遍历**************************/
        //先遍历输出左结点，再输出当前结点，最后输出右结点

        /// <summary>
        /// 中序遍历
        /// </summary>
        /// <param name="head"></param>
        public static void InOrderUnRecur(Node head)
        {
            if (head != null)
            {
                Stack<Node> stack = new Stack<Node>();
                //将首节点先入栈，然后将他一直赋值成他的左孩子，直到没有左孩子了
                //就输出当前结点，然后回到栈顶，弹出数据并输出
                //然后将结点变成右结点，保证找到所有右结点
                while (stack.Count != 0 || head != null)
                {
                    if (head != null)
                    {
                        stack.Push(head);
                        head = head.left;
                    }
                    else
                    {
                        head = stack.Pop();
                        Console.Write(head.value + " ");
                        head = head.right;
                    }
                }
            }
        }

        /// <summary>
        /// 中序遍历
        /// </summary>
        /// <param name="head"></param>
        public static void InOrderUnRecurDG(Node head)
        {
            if (head == null) return;
            InOrderUnRecurDG(head.left);
              Console.Write(head.value + " ");
            InOrderUnRecurDG(head.right);
        }

最后是后序遍历

  /***********************后序遍历**************************/
        //先遍历输出左结点，再输出右结点，最后再输出当前结点

        /// <summary>
        /// 使用两个栈实现
        /// </summary>
        /// <param name="head"></param>
        public static void PosOrderUnRecurl(Node head)
        {
            if (head != null)
            {
                Stack<Node> s1 = new Stack<Node>();
                Stack<Node> s2 = new Stack<Node>();
                //先将结点压入s1
                s1.Push(head);
                //循环遍历弹出s1的栈顶数据cu，将他的左右孩子加入s1
                //然后将cu加入s2,直到s1为空，输出s2的数据
                while (s1.Count != 0)
                {
                    head = s1.Pop();
                    s2.Push(head);
                    if (head.left != null)
                    {
                        s1.Push(head.left);
                    }

                    if (head.right != null)
                    {
                        s1.Push(head.right);
                    }
                }

                while (s2.Count != 0)
                {
                    Console.Write(s2.Pop().value + " ");
                }
            }
        }

        /// <summary>
        /// 使用递归实现后序遍历
        /// </summary>
        /// <param name="head"></param>
        public static void PosOrderUnRecur2(Node head)
        {
            if (head == null) return;
            PosOrderUnRecur2(head.left);
            PosOrderUnRecur2(head.right);
            Console.Write(head.value + " ");
        }