LeetCode 285 Binary Tree Level Order Traversal

这道题目，一刷用Queue就可以了

 

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