LeetCode: Binary Tree Zigzag Level Order Traversal

Problem:

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]
]

zigzag形遍历二叉树，使用两个栈模拟。

/**
 * 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
        vector< vector<int> > result;  
        if (root == NULL)  
            return result;  
          
        stack<TreeNode*> stk1, *curstk;
        stack<TreeNode*> stk2, *nextstk, *tmp;
        stk1.push(root);  
        curstk = &stk1;
        nextstk = &stk2;
        bool flag = false;
        vector<int> data(0);  
        while(!curstk->empty())
        {
            data.clear();  
            while(!curstk->empty())  
            {  
                root = curstk->top();
                curstk->pop();
                data.push_back(root->val);
                if (flag)
                {
                    if (root->right != NULL)
                        nextstk->push(root->right);
                    if (root->left != NULL)
                        nextstk->push(root->left);
                }
                else
                {
                    if (root->left != NULL)
                        nextstk->push(root->left);
                    if (root->right != NULL)
                        nextstk->push(root->right);
                }
               
            }  
            result.push_back(data);  
            flag = !flag;
            tmp = curstk;
            curstk = nextstk;
            nextstk = tmp;
        }
        return result;  
    }
};