二叉树的非递归遍历&递归遍历

package 实际问题;

import java.util.Stack;
public class MyTree{
    public static void main(String[] s){
        new MyTree();
    }
    public MyTree(){
        TreeNode root = init();//初始化二叉树并返回根节点
        System.out.println("递归先序遍历");
        preorder(root);
        System.out.println();
        System.out.println("递归中序遍历");
        inorder(root);
        System.out.println();
        System.out.println("递归后续遍历");
        posorder(root);
        System.out.println();
        System.out.println("非递归先序遍历");
        _preorder(root);
        System.out.println();
        System.out.println("非递归中序遍历");
        _inorder(root);
        System.out.println();
        System.out.println("非递归后续遍历");
        _posorder(root);
        System.out.println();
    }
    
    public void preorder(TreeNode root){//递归二叉树的前序遍历
        if(root != null){
            System.out.print(root.getValue());//访问节点值
            preorder(root.getLeft());
            preorder(root.getRight());
        }
    }
    public void _preorder(TreeNode root){             //非递归二叉树的前序遍历
        Stack<TreeNode> stack = new Stack<TreeNode>();//定义堆栈存储
        while(!(root == null && stack.isEmpty())){
            while (root != null) {  
                System.out.print(root.getValue());  
                stack.push(root);  
                root = root.getLeft();
            }  
            if (!stack.isEmpty()) {  
                root = stack.pop();  
                root = root.getRight();  
            }  
        }  
        
    }
    public void inorder(TreeNode root){//递归中序遍历
        if(root != null){
            inorder(root.getLeft());
            System.out.print(root.getValue());
            inorder(root.getRight());
        }
    }
    public void _inorder(TreeNode root){//非递归中序遍历
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(!(root == null && stack.isEmpty())){
            while(root != null){//先找到最深的左子树
                stack.push(root);
                root = root.getLeft();
            }
            //找到最深左子树后开始访问
            root = stack.pop();
            System.out.print(root.getValue());
            //开始寻找右子树
            root = root.getRight();
        }
    }
    public void posorder(TreeNode root){//递归后序遍历
        if(root != null){
            posorder(root.getLeft());
            posorder(root.getRight());
            System.out.print(root.getValue());
        }
    }
    //非递归的后续遍历需要两次访问节点，最后一次访问节点为准
    private int sign = 0;//得当当前访问节点的次数
    public void _posorder(TreeNode root){
        Stack stack = new Stack();//定义一个可以存放TreeNode 和 Integer的栈
        while(!(root == null && stack.isEmpty())){
            if(root != null){//找最深左子树
                stack.push(root);//将当前节点压入堆栈
                stack.push(1);//并标记当前访问节点的次数
                root = root.getLeft();
            }else{//找到最深左子树后
                while(!stack.isEmpty()){
                    sign = (Integer)stack.pop();//出栈标记
                    root = (TreeNode)stack.pop();//出栈节点
                    if(sign == 1){//地一次访问节点
                        stack.push(root);
                        stack.push(2);
                        root = root.getRight();//将节点指向右子树并开始访问指向右子树的左子树
                        break;
                    }else if(sign == 2){//当第二次出栈时访问节点
                        System.out.print(root.getValue());
                        root = null;
                    }
                }
            }
        }
    }
    public TreeNode init(){//二叉树的初始化
        TreeNode d = new TreeNode('4');
        TreeNode e = new TreeNode('5');
        TreeNode f = new TreeNode('6');
        TreeNode g = new TreeNode('7');
        TreeNode b = new TreeNode('2',d,e);
        TreeNode c = new TreeNode('3',f,g);
        TreeNode a = new TreeNode('1',b,c);
        return a;
    }
}
//建立二叉树的节点类
class TreeNode{
    public char data;
    public TreeNode left,right;
    public TreeNode(char data){
        this(data,null,null);
    }
    public TreeNode(char data,TreeNode left,TreeNode right){
        this.data = data;
        this.left = left;
        this.right = right;
    }
    public TreeNode getLeft(){//返回左子树
        return left;
    }
    public TreeNode getRight(){//返回右子树
        return right;
    }
    public char getValue(){//访问节点值
        return data;
    }
}