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果

输入：
5
6 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 stack = new Stack();
            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 stack = new Stack();
        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 stack = new Stack();
        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 queue = new LinkedBlockingQueue();
        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;
}   
}```