二叉树-树的遍历（非递归）

一、层序遍历

//层序遍历
        public static void levelOrder(Node root){
            if(root==null){
                return;
            }
            //用队列来模拟
            Queue queue=new LinkedList<>();
            //先把根节点入队列
            queue.offer(root);
            while(!queue.isEmpty()){
                //先打印根节点
                Node cur=queue.poll();
                System.out.print(cur.val+" ");
                if(cur.left!=null){
                    //左子树
                    queue.offer(cur.left);
                }
                if(cur.right!=null){
                    //左子树
                    queue.offer(cur.right);
                }
            }
        }

二、先序遍历

//先序遍历
        public static void FirstOrder(Node root) {
            if (root == null) {
                return;
            }
            Stack stack = new Stack<>();
            stack.push(root);
            while (!stack.isEmpty()) {
                Node cur = stack.pop();
                System.out.print(cur.val + " ");
                if (cur.right != null) {
                    stack.push(cur.right);
                }
                if (cur.left != null) {
                    stack.push(cur.left);
                }
            }
        }

三、中序遍历

public static List inorderTraversalhko9(Node root){
            List ret = new ArrayList<>();
            Stack stack = new Stack<>();
            Node cur = root;
            while (cur != null || !stack.isEmpty()) {
                while (cur != null) {
                    stack.push(cur);
                    cur = cur.left;
                }
                cur = stack.pop();
                ret.add(cur.val);
                cur = cur.right;
            }
            return ret;
        }

四、后序遍历

//后序遍历
        public static void postordertraversal(Node root) {
            if(root==null){
                return;
            }
            //用栈来模拟
            Stack stack = new Stack<>();
            Node prev=null;
            Node cur=root;
            //不断往左子树方向走，每走一次就将当前节点保存到栈中
            //这是模拟递归的调用
            while(true){
                //把左子树全部入栈
                while(cur!=null){
                    stack.push(cur);
                    cur=cur.left;
                }
                if(stack.isEmpty()) {
                    break;
                }
                //没有左子树了，那就取栈顶元素
                Node top=stack.peek();
                //看有没有右子树
                if(top.right==null||top.right==prev){
                    System.out.print(top.val+" ");
                    stack.pop();
                    prev=top;
                }else{
                    cur=top.right;
                }
            }
        }