Java实现二叉树的前序、中序、后序、层序遍历（非递归方法）
在上一篇博客中，实现了Java中二叉树的四种遍历方式的递归实现，接下来，在此实现Java中非递归实现二叉树的前序、中序、后序、层序遍历，在非递归实现中，借助了栈来帮助实现遍历。前序和中序比较类似，也简单一些，但是后序遍历需要两个栈来进行辅助，稍微复杂一些，层序遍历中借助了一个队列来进行实现。
同样是那棵二叉树
-
前序遍历：4 2 1 3 6 5 7 8 10
-
中序遍历：1 2 3 4 5 6 7 8 10
-
后序遍历：1 3 2 5 10 8 7 6 4
-
层序遍历：4 2 6 1 3 5 7 8 10
1、前序： 根左右
2、中序： 左根右
3、后序： 左右根
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
public class Tree<AnyType extends Comparable<? super AnyType>>
{
private static class BinaryNode<AnyType>
{
BinaryNode(AnyType theElement)
{
this(theElement, null, null);
}
BinaryNode(AnyType theElement, BinaryNode<AnyType> lt, BinaryNode<AnyType> rt)
{
element = theElement;
left = lt;
right = rt;
}
AnyType element;
BinaryNode<AnyType> left;
BinaryNode<AnyType> right;
}
private BinaryNode<AnyType> root;
public void insert(AnyType x)
{
root = insert(x, root);
}
public boolean isEmpty()
{
return root == null;
}
private BinaryNode<AnyType> insert(AnyType x, BinaryNode<AnyType> t)
{
if(t == null)
{
return new BinaryNode<>(x, null, null);
}
int compareResult = x.compareTo(t.element);
if(compareResult < 0)
{
t.left = insert(x, t.left);
}
else if(compareResult > 0)
{
t.right = insert(x, t.right);
}
else
{
;
}
return t;
}
/**
* 前序遍历
* 递归
*/
public void preOrder(BinaryNode<AnyType> Node)
{
if (Node != null)
{
System.out.print(Node.element + " ");
preOrder(Node.left);
preOrder(Node.right);
}
}
/**
* 中序遍历
* 递归
*/
public void midOrder(BinaryNode<AnyType> Node)
{
if (Node != null)
{
midOrder(Node.left);
System.out.print(Node.element + " ");
midOrder(Node.right);
}
}
/**
* 后序遍历
* 递归
*/
public void posOrder(BinaryNode<AnyType> Node)
{
if (Node != null)
{
posOrder(Node.left);
posOrder(Node.right);
System.out.print(Node.element + " ");
}
}
/*
* 层序遍历
* 递归
*/
public void levelOrder(BinaryNode<AnyType> Node) {
if (Node == null) {
return;
}
int depth = depth(Node);
for (int i = 1; i <= depth; i++) {
levelOrder(Node, i);
}
}
private void levelOrder(BinaryNode<AnyType> Node, int level) {
if (Node == null || level < 1) {
return;
}
if (level == 1) {
System.out.print(Node.element + " ");
return;
}
// 左子树
levelOrder(Node.left, level - 1);
// 右子树
levelOrder(Node.right, level - 1);
}
public int depth(BinaryNode<AnyType> Node) {
if (Node == null) {
return 0;
}
int l = depth(Node.left);
int r = depth(Node.right);
if (l > r) {
return l + 1;
} else {
return r + 1;
}
}
/**
* 前序遍历
* 非递归
*/
public void preOrder1(BinaryNode<AnyType> Node)
{
Stack<BinaryNode> stack = new Stack<>();
while(Node != null || !stack.empty())
{
while(Node != null)
{
System.out.print(Node.element + " ");
stack.push(Node);
Node = Node.left;
}
if(!stack.empty())
{
Node = stack.pop();
Node = Node.right;
}
}
}
/**
* 中序遍历
* 非递归
*/
public void midOrder1(BinaryNode<AnyType> Node)
{
Stack<BinaryNode> stack = new Stack<>();
while(Node != null || !stack.empty())
{
while (Node != null)
{
stack.push(Node);
Node = Node.left;
}
if(!stack.empty())
{
Node = stack.pop();
System.out.print(Node.element + " ");
Node = Node.right;
}
}
}
/**
* 后序遍历
* 非递归
*/
public void posOrder1(BinaryNode<AnyType> Node)
{
Stack<BinaryNode> stack1 = new Stack<>();
Stack<Integer> stack2 = new Stack<>();
int i = 1;
while(Node != null || !stack1.empty())
{
while (Node != null)
{
stack1.push(Node);
stack2.push(0);
Node = Node.left;
}
while(!stack1.empty() && stack2.peek() == i)
{
stack2.pop();
System.out.print(stack1.pop().element + " ");
}
if(!stack1.empty())
{
stack2.pop();
stack2.push(1);
Node = stack1.peek();
Node = Node.right;
}
}
}
/*
* 层序遍历
* 非递归
*/
public void levelOrder1(BinaryNode<AnyType> Node) {
if (Node == null) {
return;
}
BinaryNode<AnyType> binaryNode;
Queue<BinaryNode> queue = new LinkedList<>();
queue.add(Node);
while (queue.size() != 0) {
binaryNode = queue.poll();
System.out.print(binaryNode.element + " ");
if (binaryNode.left != null) {
queue.offer(binaryNode.left);
}
if (binaryNode.right != null) {
queue.offer(binaryNode.right);
}
}
}
public static void main( String[] args )
{
int[] input = {4, 2, 6, 1, 3, 5, 7, 8, 10};
Tree<Integer> tree = new Tree<>();
for(int i = 0; i < input.length; i++)
{
tree.insert(input[i]);
}
System.out.print("递归前序遍历 ：");
tree.preOrder(tree.root);
System.out.print("\n非递归前序遍历：");
tree.preOrder1(tree.root);
System.out.print("\n递归中序遍历 ：");
tree.midOrder(tree.root);
System.out.print("\n非递归中序遍历 ：");
tree.midOrder1(tree.root);
System.out.print("\n递归后序遍历 ：");
tree.posOrder(tree.root);
System.out.print("\n非递归后序遍历 ：");
tree.posOrder1(tree.root);
System.out.print("\n递归层序遍历：");
tree.levelOrder(tree.root);
System.out.print("\n非递归层序遍历 ：");
tree.levelOrder1(tree.root);
}
}在以上代码中，preOrder1、midOrder1、posOrder1、levelOrder1四个函数，分别代表了非递归实现的二叉树前序、中序、后序、层序遍历遍历。
