1、前言

2、思路

二叉树的层序遍历

3、代码

class Solution {
    public int[] levelOrder(TreeNode root) {
        if(root == null){
            return new int[]{};
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        List<Integer> list = new ArrayList<>();
        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            list.add(node.val);

            if(node.left != null){
                queue.add(node.left);
            }
            if(node.right != null){
                queue.add(node.right);
            }
        }
        int[] res = new int[list.size()];
        for(int i = 0; i < res.length; i++){
            res[i] = list.get(i);
        }

        return res;
    }
}