剑指offer-22：从上往下打印二叉树

题目描述

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

思路

二叉树的层序遍历，借助一个队列实现。

代码

public class Solution22 {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {

        ArrayList<Integer> arr=new ArrayList<>();
        if(root==null)
            return arr;

        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode currentNode ;
        queue.offer(root);
        while (!queue.isEmpty()) {
            currentNode = queue.poll();
            if (currentNode.left != null)
                queue.offer(currentNode.left);
            if (currentNode.right != null)
                queue.offer(currentNode.right);
            arr.add(currentNode.val);
        }
        return arr;
    }


    public static void main(String[] args) {
        
        int[] a={1,5,7,11,43,12,45,32,23,9,21};
        TreeNode tree= BeanUtil.createCompleteBinaryTree(a);
        BeanUtil.print(tree,tree.val,0);
        BeanUtil.printArr(new Solution22().PrintFromTopToBottom(tree));

    }


}