Binary Tree Zigzag Level Order Traversal

题目的意思是z字形遍历二叉树，代码有点冗余希望大牛么赐教，大概意思是要不从左到右遍历存入栈中，要不从右至左存入栈中，为了统一操作新建了一个类似头节点的树节点。

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        bool flag = true;
        vector<vector<int> > result;
        if(root==NULL)
        return result;
        vector<TreeNode*> stack;
        vector<TreeNode*> temp;
        vector<int> layer;
        TreeNode *head = (TreeNode*)malloc(sizeof(TreeNode));
        head->left = root;
        head->right = NULL;
        stack.push_back(head);
        while(!stack.empty())
        {
            temp.clear();
            layer.clear();
            if(flag)
            {
               for(vector<TreeNode*>::iterator iter = stack.end()-1;iter>=stack.begin();iter--)
               {
                   if((*iter)->left!=NULL)
                   {
                       temp.push_back((*iter)->left);
                       layer.push_back((*iter)->left->val);
                   }
                   if((*iter)->right!=NULL)
                   {
                       temp.push_back((*iter)->right);
                       layer.push_back((*iter)->right->val);
                   }
               }
            }
            else
            {
                for(vector<TreeNode*>::iterator iter = stack.end()-1;iter>=stack.begin();iter--)
               {
                   if((*iter)->right!=NULL)
                   {
                       temp.push_back((*iter)->right);
                       layer.push_back((*iter)->right->val);
                   }
                   if((*iter)->left!=NULL)
                   {
                       temp.push_back((*iter)->left);
                       layer.push_back((*iter)->left->val);
                   }
               }
            }
            stack = temp;
            if(!layer.empty())
            result.push_back(layer);
            flag = !flag;
        }
        free(head);
        head =NULL;
        return result;
    }
};