Java实现二叉树的前序、中序、后序以及层次遍历

建立一颗二叉排序树，并输出它的前序、中序、后序以及层次遍历结
输入：

56 9 1 5 8输出：6 1 5 9 8 //前序1 5 6 8 9 //中序5 1 8 9 6 //后序6 1 9 5 8 //层次

//Main函数
public static void main(String[] args){
Scanner scanner = new Scanner(System.in);
while(scanner.hasNextInt()){
int num = scanner.nextInt();
int[] array = new int[num];
for(int i = 0; i < num; i++){
array[i] = scanner.nextInt();
}
TreeNode root = new TreeNode(array[0]);
for(int i = 1; i < num; i++){
buildBST(root,array[i]);
}
DLR(root);//前序遍历
System.out.println();
LDR(root);//中序遍历
System.out.println();
LRD(root);//后序遍历
System.out.println();
levelOrder(root);//层次遍历
System.out.println();
}
}
//前序遍历非递归
public static void DLR(TreeNode node){
if(node!=null){
Stack<TreeNode> stack = new Stack<TreeNode>();
stack.push(node);
while(!stack.isEmpty()){
node = stack.pop();
System.out.print(node.val+" ");
if(node.right!=null)stack.push(node.right);
if(node.left!=null)stack.push(node.left);
}
}
}
//中序遍历非递归
public static void LDR(TreeNode node){
Stack<TreeNode> stack = new Stack<TreeNode>();
while(node!=null || !stack.isEmpty()){
if(node!=null){
stack.push(node);
node = node.left;
}else{
node = stack.pop();
System.out.print(node.val+" ");
node = node.right;
}
}
}
//后序遍历非递归
public static void LRD(TreeNode node){
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode pre = node;
while(node!=null){
while(node.left!=null){
stack.push(node);
node = node.left;
}
while(node!=null && (node.right==null || node.right==pre)){
System.out.print(node.val+" ");
pre = node;
if(stack.isEmpty()) return;
node = stack.pop();
}
stack.push(node);
node = node.right;
}
}
//层次遍历非递归
public static void levelOrder(TreeNode node){
Queue<TreeNode> queue = new LinkedBlockingQueue<TreeNode>();
queue.offer(node);
while(!queue.isEmpty()){
node = queue.poll();
System.out.print(node.val+" ");
if(node.left!=null)queue.add(node.left);
if(node.right!=null)queue.add(node.right);
}
}
//构建二叉排序树
public static void buildBST(TreeNode node,int val){
if(node.val == val) return ;
if(val < node.val){
if(node.left==null) node.left = new TreeNode(val);
else buildBST(node.left,val);
}else{
if(node.right==null) node.right = new TreeNode(val);
else buildBST(node.right,val);
}
}


//二叉树节点结构
class TreeNode{
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val){
this.val = val;
}
}```