https://www.acwing.com/problem/content/43/

今天写这题，bfs层序遍历去遍历，代码如下

java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        
        if (root == null) {
            return res;
        }
        
        
        Queue<TreeNode> queue = new LinkedList<>();
        Boolean isLeft = true;
        
        queue.offer(root);
        while(!queue.isEmpty()){
            
            List<Integer> currentLevel = new ArrayList<>();
            
            Integer size = queue.size();
            
            for(int i = 0;i < size; i++) {
                
                TreeNode node = queue.poll();
                currentLevel.add(node.val);
                
                if(node.left != null) {
                    queue.add(node.left);
                }
                
                if(node.right != null) {
                    queue.add(node.right);
                }
                
                
            }
            
            
            if(!isLeft) {
                 Collections.reverse(currentLevel);
            }
            
            res.add(currentLevel);
            isLeft = !isLeft;
            
            
        }
        
        return res;
    }
}