102. 二叉树的层序遍历

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()) {
                List<Integer> temp = new ArrayList<>();

                int levelSize = queue.size();
                //每次将当前层的结点从左向右取出，将下一层的结点从左向右放进去
                for (int i = 0; i < levelSize; i++) {
                    TreeNode curr = queue.poll();
                    temp.add(curr.val);

                    if (curr.left != null) {
                        queue.offer(curr.left);
                    }
                    if (curr.right != null) {
                        queue.offer(curr.right);
                    }
                }
                res.add(temp);
            }

            return res;
        }
    }