二叉树的遍历是解决树类问题的关键，二叉树的遍历分为递归和非递归。一般来说，递归的非递归的简单许多，但是一般要求能够写出非递归的代码，并且讲清楚非递归的算法思想。
1.二叉树的数据结构
image.png
package com.zhoujian.solutions.dataStructure.tree;
/**
* @author zhoujian 2018/8/18
*/
public class Node {
public int value;
public Node left;
public Node right;
//构造器
public Node(int value) {
this.value = value;
}
}
这是二叉树的节点的数据类型结构。
2.二叉树的遍历（递归）
package com.zhoujian.solutions.dataStructure.tree;
/**
* @author zhoujian123@hotmail.com 2018/4/30 21:27
*
* 二叉树的遍历递归和非递归方式
*/
public class BinaryTree {
public static void preOrder(Node head){
if (head == null) {
return;
}
System.out.print(head.value);
preOrder(head.left);
preOrder(head.right);
}
public static void inOrder(Node head){
if (head == null) {
return;
}
preOrder(head.left);
System.out.print(head.value);
preOrder(head.right);
}
public static void postOrder(Node head){
if (head == null) {
return;
}
preOrder(head.left);
preOrder(head.right);
System.out.print(head.value);
}
public static void main(String[] args) {
//根节点是5
Node node =new Node(1);
node.left = new Node(2);
node.right = new Node(3);
node.left.left = new Node(4);
node.left.right = new Node(5);
node.right.left = new Node(6);
node.right.right = new Node(7);
System.out.println("先序遍历");
preOrder(node);
System.out.println();
System.out.println("中序遍历");
inOrder(node);
System.out.println();
System.out.println("后序遍历");
postOrder(node);
}
}
3.二叉树的遍历（非递归）
package com.zhoujian.solutions.dataStructure.tree;
import java.util.Stack;
/**
* @author zhoujian 2018/8/18
*/
public class BinaryTreeTest {
//前序遍历的思想：借助一个栈，先放入右孩子，在放做孩子。在从栈顶弹出栈顶元素，查看当前元素有没有左右孩 //子。
public static void proOrderunRecur(Node head){
if (head != null) {
Stack<Node> stack = new Stack<Node>();
stack.add(head);
while (!stack.isEmpty()){
//不为空先弹出栈顶元素
head = stack.pop();
System.out.print(head.value);
//先看有没有右孩子
if(head.right !=null){
stack.push(head.right);
}
//在看有没有左孩子
if(head.left != null){
stack.push(head.left);
}
}
}
System.out.println();
}
//中序遍历：借助一个栈，若该节点不是叶子节点，就一直循环线找到最左的孩子，直到叶子节点，然后弹出栈顶元 //素，查看该节点有没有右孩子。若有，则将右孩子压入栈中。若没有，则继续弹出栈顶元素。
public static void inOrderunRecur(Node head){
if (head != null) {
Stack<Node> stack = new Stack<Node>();
while (!stack.isEmpty() || head != null){
//判断是否为叶子节点
if (head != null){
//一直循环到最左的节点
stack.push(head);
head = head.left;
}else {
//将栈顶元素弹出
head = stack.pop();
System.out.print(head.value);
//把此时的右节点加入栈中
head = head.right;
}
}
}
}
public static void posOrderunRecur(Node head){
if (head != null) {
Stack<Node> stack1 = new Stack<Node>();
Stack<Node> stack2 = new Stack<Node>();
//先将栈顶元素压入栈中
stack1.push(head);
while (!stack1.isEmpty()){
//从stack1中弹出栈顶元素，放入到stack2中
head = stack1.pop();
stack2.push(head);
if (head.left != null) {
stack1.push(head.left);
}
if (head.right != null) {
stack1.push(head.right);
}
}
//从栈中依次打印出来，打印出来的顺序就是后序遍历
while (!stack2.isEmpty()){
System.out.print(stack2.pop().value);
}
}
}
public static void main(String[] args) {
//根节点是5
Node node =new Node(1);
node.left = new Node(2);
node.right = new Node(3);
node.left.left = new Node(4);
node.left.right = new Node(5);
node.right.left = new Node(6);
node.right.right = new Node(7);
System.out.println("前序遍历");
proOrderunRecur(node);
System.out.println("中序遍历");
inOrderunRecur(node);
System.out.println();
System.out.println("后序遍历");
posOrderunRecur(node);
}
}
后序遍历有点复杂：
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