给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回锯齿形层次遍历如下：

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

/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var zigzagLevelOrder = function(root) {
  if (!root) return [];   
  const items = [];
  let isOdd = true;
  let levelNodes = [];
  
  const queue = [root, null];


  while(queue.length > 0) {
      const t = queue.shift();

      if (t) {
          levelNodes.push(t.val)
          if (t.left) {
            queue.push(t.left)
          }
          if (t.right) {
            queue.push(t.right)
          }
      } else {
        if (!isOdd) {
          levelNodes = levelNodes.reverse();
        }
        items.push(levelNodes)
        levelNodes = [];
        isOdd = !isOdd;
        if (queue.length > 0) {
            queue.push(null);
        }
      }
  }

  return items
    
};

/**
 * 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>> zigzagLevelOrder(TreeNode* root) {
        auto ret = vector<vector<int>>();
        if (root == nullptr) return ret;
        auto queue = vector<const TreeNode*>{root};
        auto isOdd = true;
        while (!queue.empty()) {
            auto sz = queue.size();
            auto level = vector<int>();
            for (auto i = 0; i < sz; ++i) {
                auto n = queue.front();
                queue.erase(queue.begin());
                if (isOdd) level.push_back(n->val);
                else level.insert(level.begin(), n->val);
                if (n->left != nullptr) queue.push_back(n->left);
                if (n->right != nullptr) queue.push_back(n->right);
            }
            isOdd = !isOdd;
            ret.push_back(level);
        }
        return ret;
    }
};

class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        auto ret = vector<vector<int>>();
        zigzagLevelOrder(root, 0, ret);
        return ret;
    }
private:
    void zigzagLevelOrder(const TreeNode* root, int level, vector<vector<int>>& ret) {
        if (root == nullptr || level < 0) return;
        if (ret.size() <= level) {
            ret.push_back(vector<int>());
        }
        if (level % 2 == 0) ret[level].push_back(root->val);
        else ret[level].insert(ret[level].begin(), root->val);
        zigzagLevelOrder(root->left, level + 1, ret);
        zigzagLevelOrder(root->right, level + 1, ret);
    }
};