Given a binary tree, return the zigzag level order traversal of its nodes‘ values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   /   9  20
    /     15   7

return its zigzag level order traversal as:

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

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    std::vector<std::vector<int> > zigzagLevelOrder(TreeNode *root) {
        dfs(root,0);
		for (int i = 0; i < res.size(); i++)
		{
			if(i % 2)
				std::reverse(res[i].begin(),res[i].end());
		}
		return res;
    }
private:
	std::vector<std::vector<int>> res;
	void dfs(TreeNode *root,int level)
	{
		if(root == NULL) return;
		if(res.size() == level)
		{
			res.push_back(std::vector<int>());
		}
		res[level].push_back(root->val);
		dfs(root->left,level+1);
		dfs(root->right,level+1);
	}
};

leetcode - Binary Tree Zigzag Level Order Traversal