二叉树前序/中序/后序非递归实现解决方法

package com.bitree;import java.util.*;public class BiTree {    public BiTree(){        Node node6 = new Node(6, null, null);        Node node7 = new Node(7, null, null);        Node node5 = new Node(5, node6, node7);        Node node4 = new Node(4, null, node5);        Node node3 = new Node(3, null, null);        Node node2 = new Node(2, node4, null);        this.root = new Node(1, node2, node3);    }        public void PreOrder(){        Stack<Node> s = new Stack<Node>();        s.push(this.root);        while (!s.isEmpty())        {            Node node = s.pop();            System.out.print(node.getItem());            if (node.getRightChild() != null){                s.push(node.getRightChild());            }            if (node.getLeftChild() != null){                s.push(node.getLeftChild());            }        }    }    public void InOrder(){        Stack<Node> s= new Stack<Node>();        Node node = this.root;        while (node != null || !s.isEmpty()){            while (node != null){                s.push(node);                node = node.getLeftChild();            }                        if (!s.isEmpty()){                node = s.pop();                System.out.print(node.getItem());                node = node.getRightChild();            }        }    }    public void PostOrder(){        Stack<Node> s = new Stack<Node>();        Stack<Node> sPre = new Stack<Node>();        Node node = this.root;        Node nodePrev = null;        while (node != null || !s.isEmpty()){            while (node != null){                s.push(node);                node = node.getLeftChild();            }                        if (!s.isEmpty()){                node = s.peek();                if (!sPre.isEmpty())                {                    nodePrev = sPre.peek();                    if (nodePrev == node)                        sPre.pop();                }                if (node.getRightChild() == null || node == nodePrev){                    node = s.pop();                    System.out.print(node.getItem());                    node = null;                }                else{                    sPre.push(node);                    node = node.getRightChild();                }            }        }    }        private Node root;        private class Node{        public Node(int n, Node lNode, Node rNode){            this.makeNode(n, lNode, rNode);        }        public void makeNode(int n, Node lNode, Node rNode){            this.leftChild = lNode;            this.rightChild = rNode;            this.nItem = n;        }        public Node getLeftChild(){            return this.leftChild;        }        public Node getRightChild(){            return this.rightChild;        }        public int getItem(){            return this.nItem;        }        private int nItem;        private Node leftChild;        private Node rightChild;    }}