思路:

• 层序遍历BFS
• 用一个队列来维护每一层的元素

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if(root == null)
            return ans;
        Queue<TreeNode> que = new LinkedList<>();
        que.offer(root);
        while(!que.isEmpty()){
            int size = que.size();
            List<Integer> path = new ArrayList<>();
            for(int i = 0 ; i < size ; i++){
                TreeNode cur = que.poll();
                path.add(cur.val);
                if(cur.left != null)
                    que.offer(cur.left);
                if(cur.right != null)
                    que.offer(cur.right);
            }
            ans.add(new ArrayList<>(path));

        }
        return ans;
    }
}