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)
        return result;
    
    stack<TreeNode *> q1;
    stack<TreeNode *> q2;
    q1.push(root);
    while(q1.size() > 0 || q2.size() > 0)
    {
       vector<int> v1;
        while(q1.size() > 0)
        {
            TreeNode *top = q1.top();
            
            q1.pop();
            v1.push_back(top->val);
            
           
            if(top->left)
                q2.push(top->left);
            
            if(top->right)
                q2.push(top->right); 

        }
        
        if(v1.size() > 0)
        {
            result.push_back(v1);
        }
      
        vector<int> v2;       
        while(q2.size() > 0)
        {
            TreeNode *top = q2.top();
            
            q2.pop();
            v2.push_back(top->val);
            
         
            if(top->right)
                q1.push(top->right);
            
            if(top->left)
                q1.push(top->left);  
         }
        
        if(v2.size() > 0)
        {
            result.push_back(v2);}
     }
}
};