非递归方式 实现 前中后序遍历二叉树

public class Code03_UnRecursiveTraversalBT_chang_done {
    public static class Node {        public int value;        public Node left;        public Node right;
        public Node(int v) {            value = v;        }    }
    /* 按照前序遍历的逻辑 先来head再找左子树去弹头节点     * 来一个节点  利用压栈弹栈来存储弹出节点 控制顺序     * head先弹出, 压入右左     * 下一轮 弹head 压入右左  直到左右节点没有子节点 继续弹出左右节点     *     *     * => 头右左 就很容易    再用一个stack来暂存要打印的  最后再打印 => 左右头  后序遍历     */    public static void pre(Node head) {        System.out.print("pre-order: ");        if (head != null) {            //建立stack            Stack<Node> stack = new Stack<>();            stack.push(head);            //提前处理 便于统一while处理            while (!stack.isEmpty()) {                //弹出head  打印                Node tempHead = stack.pop();                System.out.print(tempHead.value + " ");                //压入右左                if (tempHead.right != null) {                    stack.push(tempHead.right);                }                if (tempHead.left != null) {                    stack.push(tempHead.left);                }            }        }        System.out.println();    }    //整棵树按左边界分解 (见笔记图) 符合中序遍历的逻辑: 左节点 中节点 右节点    //    public static void in(Node cur) {        System.out.print("in-order: ");        if(cur!=null){            Stack<Node> stack = new Stack<>();            while(!stack.isEmpty() || cur != null){  //未到到最右下节点的条件  根据左边界分解, 到最右下时 俩东西为空了                //统一压入左边界(包含了多个左节点和头节点 且他们交叉压入的)                if(cur!=null){                    stack.push(cur);                    cur = cur.left;                }else{                    //到达最左下节点后 cur为null  弹出自己                    Node tempNode = stack.pop();                    System.out.print(tempNode.value+" ");                    cur = tempNode.right; //为空 让下一轮弹头节点                }            }        }        System.out.println();    }
    public static void pos1(Node head) {        System.out.print("pos1-order: ");        //准备两个stack 第二个用来存储pre变种要打印的 最后才统一打印        Stack<Node> stack1 = new Stack<>();        Stack<Node> stack2 = new Stack<>();        if(head != null){            stack1.push(head);            while(!stack1.isEmpty()){                Node tempNode = stack1.pop();                stack2.push(tempNode);                if(tempNode.left !=null){ //pre变种  头左右 -> 头右左                    stack1.push(tempNode.left);                }                if(tempNode.right !=null){                    stack1.push(tempNode.right);                }            }        }
        while(!stack2.isEmpty()){            System.out.print(stack2.pop().value+" ");        }        System.out.println();    }
    public static void main(String[] args) {        Node head = new Node(1);        head.left = new Node(2);        head.right = new Node(3);        head.left.left = new Node(4);        head.left.right = new Node(5);        head.right.left = new Node(6);        head.right.right = new Node(7);
        pre(head);        System.out.println("====头左右====");        in(head);        System.out.println("====左头右====");        pos1(head);        System.out.println("====左右头====");       // pos2(head);        System.out.println("========");    }
}