非递归方式实现二叉树的三种遍历

非递归方式实现二叉树的先序、中序、后序遍历：

1. 任何递归函数都可以改成非递归
2. 自己设计压栈来实现

先序遍历：

1. 弹出就打印
2. 如果有右孩子，压入右孩子
3. 如果有左孩子，压入左孩子

后序遍历：需要用到两个栈

1. 额外申请一个栈s2，弹出后放入s2中
2. 如果有左孩子，压入左孩子
3. 如果有右孩子，压入右孩子

中序遍历：

1. 整条左边界依次入栈
2. 条件1不满足，弹出就打印，并来到弹出结点的右树继续执行条件1

后续遍历：只需要用到一个栈，另外设置两个变量h和c，h记录之前打印的结点的位置，c记录栈顶的位置。

package com.harrison.class07;

import java.util.Stack;

public class Code02_UnRecursiveTraversalBT {
    
    
	public static class Node{
    
    
		public int value;
		public Node left;
		public Node right;
		
		public Node(int v) {
    
    
			value=v;
		}
	}
	
	public static void pre(Node head) {
    
    
		System.out.println("pre-order：");
		if(head!=null) {
    
    
			Stack<Node> s=new Stack<>();
			s.push(head);
			while(!s.isEmpty()) {
    
    
				head=s.pop();
				System.out.print(head.value+" ");
				if(head.right!=null) {
    
    
					s.push(head.right);
				}
				if(head.left!=null) {
    
    
					s.push(head.left);
				}
			}
		}
		System.out.println();
	}
	
	public static void pos1(Node head) {
    
    
		System.out.println("pos-order：");
		if(head!=null) {
    
    
			Stack<Node> s1=new Stack<>();
			Stack<Node> s2=new Stack<>();
			s1.push(head);
			while(!s1.isEmpty()) {
    
    
				head=s1.pop();
				s2.push(head);
				if(head.left!=null) {
    
    
					s1.push(head.left);
				}
				if(head.right!=null) {
    
    
					s1.push(head.right);
				}
			}
			while(!s2.isEmpty()) {
    
    
				System.out.print(s2.pop().value+" ");
			}
		}
		System.out.println();
	}
	
	public static void in(Node head) {
    
    
		System.out.println("in-order：");
		if(head!=null) {
    
    
			Stack<Node> s=new Stack<>();
			while(!s.isEmpty() || head!=null) {
    
    
				if(head!=null) {
    
    
					s.push(head);
					head=head.left;
				}else {
    
    
					head=s.pop();
					System.out.print(head.value+" ");
					head=head.right;
				}
			}
		}
		System.out.println();
	}
	
	public static void pos2(Node h) {
    
    
		System.out.println("pos-order：");
		if(h!=null) {
    
    
			Stack<Node> stack=new Stack<>();
			stack.push(h);
			Node c=null;
			while(!stack.isEmpty()) {
    
    
				c=stack.peek();
				if(c.left!=null && h!=c.left && h!=c.right) {
    
    
					stack.push(c.left);
				}else if(c.right!=null && h!=c.right) {
    
    
					stack.push(c.right);
				}else {
    
    
					System.out.print(stack.pop().value+" ");
					h=c;
				}
			}
		}
		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("========");
		in(head);
		System.out.println("========");
		pos1(head);
		System.out.println("========");
		pos2(head);
		System.out.println("========");
	}

}