[Leetcode] Binary Tree Zigzag Level Order Traversal

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本文深入解析了zigzag形遍历二叉树的算法实现，通过使用两个栈交替存储的方法，实现了从左到右再到左的层次遍历。详细展示了算法的核心逻辑与步骤，并通过示例代码进行了说明。

题目：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}

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

思路：zigzag形遍历二叉树，使用两个栈交替存储。

vector<vector<int> > zigzagLevelOrder(TreeNode *root) {  
        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;    
    }