什麼是二叉樹?
定義:二叉樹是每個結點最多有兩個子樹的樹結構。通常子樹被稱作“左子樹”和“右子樹”。
那什麼是遍歷?
遍歷:所謂遍歷(Traversal)是指沿着某條搜索路線,依次對樹中每個結點均做一次且僅做一次訪問。
二叉樹的遍歷方式有以下幾種:
1.前序遍歷:先訪問根節點——左子樹——右子樹。
2.中序遍歷:先訪問左子樹——根節點——右子樹,按照這個順序。
3.後序遍歷:和前面差不多,先訪問樹的左子樹——右子樹——根節點。按層遍歷:把一棵樹從上到下,從左到右依次寫出來。
4.層序遍歷:將二叉樹的每一層分別遍歷,直到最後的葉子節點被全部遍歷完
現有二叉樹如下,其通過各種遍歷方式得到的遍歷結果如下:
前序:0137849256
中序:7381940526
後序:7839415620
層序:0123456789
這裏先介紹前,中,後三種遍歷方式,最基本的實現方式就是通過遞歸實現:
首先編寫一個TreeNode類,並添加一個構造二叉樹的方法
package study.main.tree.binarytree;
/**
* @author : [email protected]
* @date : 2019/10/31 20:47
*/
public class TreeNode{
private int data;
private TreeNode leftNode;
private TreeNode rightNode;
public TreeNode(int data){
this.data = data;
}
public void setData(int data){
this.data = data;
}
public int getData(){
return this.data;
}
public void setLeftNode(TreeNode node){
this.leftNode = node;
}
public TreeNode getLeftNode(){
return this.leftNode;
}
public void setRightNode(TreeNode node){
this.rightNode = node;
}
public TreeNode getRightNode(){
return this.rightNode;
}
/**
* 構造二叉樹
* @return TreeNode
*/
public TreeNode getTree(){
TreeNode[] nodes = new TreeNode[10];
for (int i = 0;i < nodes.length;i++){
nodes[i] = new TreeNode(i);
}
nodes[0].leftNode = nodes[1];
nodes[0].rightNode = nodes[2];
nodes[1].leftNode = nodes[3];
nodes[1].rightNode = nodes[4];
nodes[2].leftNode = nodes[5];
nodes[2].rightNode = nodes[6];
nodes[3].leftNode = nodes[7];
nodes[3].rightNode = nodes[8];
nodes[4].leftNode = nodes[9];
return nodes[0];
}
}
編寫實現類
package study.main.tree.binarytree;
/**
* @author : [email protected]
* @date : 2019/10/31 20:47
*/
public class BinaryTree{
public static void main(String[] args){
TreeNode treeNode = new TreeNode(10);
//生成初始二叉樹
TreeNode node = treeNode.getTree();
BinaryTree binaryTree = new BinaryTree();
binaryTree.qianxu(node);
System.out.println("");
binaryTree.zhongxu(node);
System.out.println("");
binaryTree.houxu(node);
System.out.println(" ");
}
private void qianxu(TreeNode node){
if (null == node){
return;
}
System.out.print(node.getData()+" ");
qianxu(node.getLeftNode());
qianxu(node.getRightNode());
}
private void zhongxu(TreeNode node){
if (null == node){
return;
}
zhongxu(node.getLeftNode());
System.out.print(node.getData()+" ");
zhongxu(node.getRightNode());
}
private void houxu(TreeNode node){
if (null == node){
return;
}
houxu(node.getLeftNode());
houxu(node.getRightNode());
System.out.print(node.getData()+" ");
}
}
除了使用遞歸,還可以用java的棧stack來實現三種遍歷方式
什麼是棧?
棧是一種數據項按序排列的數據結構,只能在一端(稱爲棧頂(top))對數據項進行插入和刪除。特點:後進先出
package study.main.tree.binarytree;
import java.util.Stack;
/**
* @author : [email protected]
* @date : 2019/11/3 17:54
*/
public class BinaryTreeByStack {
public static void main(String[] args){
TreeNode treeNode = new TreeNode(10);
TreeNode node = treeNode.getTree();
BinaryTreeByStack binaryTreeByStack = new BinaryTreeByStack();
binaryTreeByStack.qianxu(node);
System.out.println(" ");
binaryTreeByStack.zhongxu(node);
System.out.println(" ");
binaryTreeByStack.houxu(node);
}
private void qianxu(TreeNode node){
Stack<TreeNode> stack = new Stack<>();
TreeNode root = node;
while (null != root || !stack.isEmpty()){
if (null != root){
System.out.print(root.getData()+" ");
stack.push(root);
root = root.getLeftNode();
} else {
root = stack.pop().getRightNode();
}
}
}
private void zhongxu(TreeNode node){
Stack<TreeNode> stack = new Stack<>();
TreeNode root = node;
while (null != root || !stack.isEmpty()){
if (null != root){
stack.push(root);
root = root.getLeftNode();
} else {
root = stack.pop();
System.out.print(root.getData()+" ");
root = root.getRightNode();
}
}
}
private void houxu(TreeNode node){
if (null == node) {
return;
}
TreeNode root = node;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
stack.push(root);
while (!stack.isEmpty()){
root = stack.pop();
if (!stack.isEmpty() && root == stack.peek()){
if (null != root.getRightNode()){
stack.push(root.getRightNode());
stack.push(root.getRightNode());
}
if (null != root.getLeftNode()){
stack.push(root.getLeftNode());
stack.push(root.getLeftNode());
}
} else{
System.out.print(root.getData()+" ");
}
}
}
}
講完前,中,後三種遍歷方式,這裏再說一下層序遍歷,它是最接近我們思維的一種遍歷方式,這裏用java的隊列Queue來實現它
隊列是一種特殊的線性表,它只允許在表的前端進行刪除操作,而在表的後端進行插入操作。
LinkedList類實現了Queue接口,因此我們可以把LinkedList當成Queue來用。
package study.main.tree.binarytree;
import java.util.LinkedList;
import java.util.Queue;
/**
* @author : [email protected]
* @date : 2019/11/10 23:33
*/
public class BinaryTreeByQueue {
public static void main(String[] args){
TreeNode treeNode = new TreeNode(10);
//生成初始二叉樹
TreeNode node = treeNode.getTree();
BinaryTreeByQueue binaryTree = new BinaryTreeByQueue();
binaryTree.cengxu(node);
}
private void cengxu(TreeNode node){
if (null == node){
return;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(node);
while (!queue.isEmpty()){
node = queue.poll();
if (null != node.getLeftNode()){
queue.offer(node.getLeftNode());
}
if (null != node.getRightNode()){
queue.offer(node.getRightNode());
}
System.out.print(node.getData()+" ");
}
}
}