题目描述

BFS

JAVA 代码

class Solution {
    public List<Integer> printFromTopToBottom(TreeNode root) {
        ArrayList<TreeNode> nodeList = new ArrayList<>();
        if (root == null){
            return new ArrayList<>();
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()){
            TreeNode node = queue.poll();
            nodeList.add(node);
            if (node.left != null){
                queue.add(node.left);
            }
            if (node.right != null){
                queue.add(node.right);
            }
        }
        return nodeList.stream().map((n) -> n.val).collect(Collectors.toList());
    }
}