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;

    }

};