[数据结构与算法]实现一颗二叉树的先中后序遍历，使用递归和非递归方式遍历

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递归版：

public class TreeNode {

    public static class Tree{
        private int number;
        private Tree leftNode;
        private Tree rightNode;

        public Tree(int number, Tree leftNode, Tree rightNode) {
            this.number = number;
            this.leftNode = leftNode;
            this.rightNode = rightNode;
        }
    }


    public static void creadTreeNode(){
        Tree tree7 =new Tree(7,null,null);
        Tree tree6 =new Tree(6,null,null);
        Tree tree5 =new Tree(5,null,null);
        Tree tree4 =new Tree(4,null,null);
        Tree tree3 =new Tree(3,tree6,tree7);
        Tree tree2 =new Tree(2,tree4,tree5);
        Tree tree1 =new Tree(1,tree2,tree3);
        prTreeNode(tree1);
    }


    public static void prTreeNode(Tree node){
        if(null== node){
            return;
        }
        System.out.println(node.number);  //先序遍历
        prTreeNode(node.leftNode);
        //System.out.println(node.number); 中序遍历
        prTreeNode(node.rightNode);
        //System.out.println(node.number);后序遍历

    }

    public static void main(String[] args) {
        creadTreeNode();
    }

}

非递归：

public class TreeNode {

    public static class Tree {
        private int number;
        private Tree leftNode;
        private Tree rightNode;

        public Tree(int number, Tree leftNode, Tree rightNode) {
            this.number = number;
            this.leftNode = leftNode;
            this.rightNode = rightNode;
        }
    }


    public static void creadTreeNode() {
        Tree tree7 = new Tree(7, null, null);
        Tree tree6 = new Tree(6, null, null);
        Tree tree5 = new Tree(5, null, null);
        Tree tree4 = new Tree(4, null, null);
        Tree tree3 = new Tree(3, tree6, tree7);
        Tree tree2 = new Tree(2, tree4, tree5);
        Tree tree1 = new Tree(1, tree2, tree3);
        preTreeNode(tree1);
        centerTreeNode(tree1);
        endTreeNode(tree1);
    }

    /**
     * 先序遍历：当前节点不为空，将当前节点压入栈。
     * 1、如果栈不为空，则弹出一个节点并打印该节点信息，如果该节点右子节点不为空，将其压入栈中，如果该节点左子节点不为空，将其压入栈中。
     *
     * @param node
     */

    public static void preTreeNode(Tree node) {
        if (null != node) {
            Stack<Tree> stack = new Stack<>();//创建一个栈
            stack.push(node);
            while (!stack.isEmpty()) {
                Tree pop = stack.pop();
                System.out.print(pop.number + " ");
                if (null != pop.rightNode) {//栈是先进后出结构，所以先判断右节点是否为空，弹出式先弹出左节点
                    stack.push(pop.rightNode);
                }
                if (null != pop.leftNode) {
                    stack.push(pop.leftNode);
                }
            }

        }
        System.out.println();
    }

    /**
     * 中序遍历：1、当前节点不为空，将当前节点放入栈中，取当前节点左子节点，再次进行第一步。
     * 2、如果当前节点为空且栈不为空，则从栈中弹出一个节点并打印该节点信息，取当前节点右子节点，再次进行第一步。
     *
     * @param node
     */
    public static void centerTreeNode(Tree node) {
        if (null != node) {
            Stack<Tree> stack = new Stack<>();
            while (!stack.isEmpty() || null != node) {
                if (null != node) {
                    stack.push(node);
                    node = node.leftNode;
                } else {
                    Tree pop = stack.pop();
                    System.out.print(pop.number + " ");
                    node = pop.rightNode;
                }
            }
        }
        System.out.println();
    }

    /**
     * 后序遍历：当前节点不为空，将当前节点压入栈。
     *       1、如果栈不为空，则弹出一个节点并将该节点信息放入辅助栈，如果该节点左子节点不为空，将其压入栈中，如果该节点右子节点不为空，将其压入栈中。
     *      
     * @param node
     */
    public static void endTreeNode(Tree node) {
        if (null != node) {
            Stack<Tree> stack = new Stack<>();
            stack.push(node);
            Stack<Tree> fuzhu = new Stack<>();
            while (!stack.isEmpty()) {
                 node = stack.pop();
                fuzhu.push(node);
                if (null != node.leftNode) {
                    stack.push(node.leftNode);
                }
                if (null != node.rightNode) {
                    stack.push(node.rightNode);
                }
            }
            while (!fuzhu.isEmpty()) {
                System.out.print(fuzhu.pop().number + " ");
            }
        }
        System.out.println();
    }

    public static void main(String[] args) {
        creadTreeNode();
    }

}