剑指 Offer 32

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()){
            List<Integer> list = new ArrayList<>();
            int count = queue.size();
            while(count>0){
                TreeNode node = queue.poll();
                list.add(node.val);
                count--;
                if(node.left!=null) queue.add(node.left);
                if(node.right!=null) queue.add(node.right);
            }
            res.add(list);
        }
        return res;
    }
}

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执行用时0ms的例子很巧妙，学习到了。