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    3
   / \
  9  20
    /  \
   15   7

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[
  [3],
  [9,20],
  [15,7]
]

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root){
            return res;
        }
        queue<TreeNode*> q;  //queue for treenode which vist with inorder
        // init queue with root
        q.push(root);
        q.push(NULL); // level flag
        vector<int> cur_level;  //
        while(!q.empty()){
            // deque
            auto n = q.front(); q.pop();

            if(n) {
                cur_level.push_back(n->val);
                if(n->left){
                    q.push(n->left);
                }
                if(n->right){
                    q.push(n->right);
                }
            } else {
                res.push_back(cur_level); cur_level.resize(0);
                if(q.size()>0){
                    q.push(NULL);
                }

            }
        }

        return res;
    }
};

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// @lc code=start
// Definition for a binary tree node.
// #[derive(Debug, PartialEq, Eq)]
// pub struct TreeNode {
//   pub val: i32,
//   pub left: Option<Rc<RefCell<TreeNode>>>,
//   pub right: Option<Rc<RefCell<TreeNode>>>,
// }
//
// impl TreeNode {
//   #[inline]
//   pub fn new(val: i32) -> Self {
//     TreeNode {
//       val,
//       left: None,
//       right: None
//     }
//   }
// }
use std::cell::RefCell;
use std::collections::VecDeque;
use std::rc::Rc;
impl Solution {
    /// ## 解题思路
    /// - 队列
    pub fn level_order(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<Vec<i32>> {
        let mut res = Vec::new(); //result
        let mut q = VecDeque::new();
        root.map(|root| {
            q.push_back(root.clone());
            while !q.is_empty() {
                let mut cur_level_vals = Vec::new();
                for _ in 0..q.len() {
                    if let Some(node) = q.pop_front() {
                        cur_level_vals.push(node.borrow().val);
                        node.borrow().left.clone().map(|left| {
                            q.push_back(left);
                        });
                        node.borrow().right.clone().map(|right| {
                            q.push_back(right);
                        });
                    }
                }

                res.push(cur_level_vals);
            }
        });

        res
    }
}
// @lc code=end