Java二叉树的层序遍历leetcode

给你二叉树的根节点 root ，返回其节点值的 层序遍历 。 （即逐层地，从左到右访问所有节点）。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> list1 = new ArrayList<List<Integer>>();
        if(root == null){
            return list1;
        }
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);

        TreeNode temp = new TreeNode();
        while(!queue.isEmpty()){
            List<Integer> list2 = new ArrayList<Integer>();
            int size = queue.size();
            for(int i = 0;i < size;i++){
                temp = queue.poll();
                list2.add(temp.val);
                if(temp.left != null){
                    queue.add(temp.left);
                }
                if(temp.right != null){
                    queue.add(temp.right);
                }
            }
            list1.add(list2);
        }
        return list1;
    }
}