Binary Tree Zigzag Level Order Traversal

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) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) 
    {
        vector<vector<int> > result;
        if(root==NULL) return result;
        vector<TreeNode*> v;
        v.push_back(root);
        
        vector<int> vals;
        bool asc=true;
        
        while(v.size()>0)
        {
            vals.clear();
            vector<TreeNode*> vnew;
            for(int i=0;i<v.size();i++)
            {
                vals.push_back(v[i]->val);
                if(v[i]->left!=NULL) vnew.push_back(v[i]->left);
                if(v[i]->right!=NULL) vnew.push_back(v[i]->right);
            }
            if(!asc) 
                for(int i=0;i<vals.size()/2;i++)
                {
                    int temp=vals[i];
                    vals[i]=vals[vals.size()-1-i];
                    vals[vals.size()-1-i]=temp;
                }
            result.push_back(vals);
            asc=!asc;
            v=vnew;
        }
        return result;
    }
};