[Leetcode] Binary tree Zigzag level order traversal二叉树Z形层次遍历

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]
]

confused what"{1,#,2,3}"means? > read more on how binary tree is serialized on OJ.

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

   1
  / 
 2   3
    /
   4
    
     5

The above binary tree is serialized as"{1,2,3,#,#,4,#,#,5}".

题目要求按Z字形以此输出每层，主体还是层次遍历。

思路一：只在Binary tree level order traversal的基础上增加对层数的奇偶的判断，然后依此来决定是否反转当前行即可；

/**
 * 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>> levelOrderBottom(TreeNode* root)
    {
        vector<vector<int>> res;
        queue<TreeNode *> Q;
        if(root)    Q.push(root);
        size_t level=1;     //第level层，初始为第一层
        while( !Q.empty())
        {
            int count=0;
            int levCount=Q.size();
            vector<int> levNode;

            while(count<levCount)
            {
                TreeNode *curNode=Q.front();
                Q.pop();
                levNode.push_back(curNode->val);
                if(curNode->left)
                    Q.push(curNode->left);
                if(curNode->right)
                    Q.push(curNode->right);
                count++;
            }
            if(level%2 ==0)     //偶数层，反转levNode再压入res
                reverse(levNode.begin(),levNode.end());
            res.push_back(levNode);
            level++;
        }
        return res;
    }
};

 思路二：

利用队列，在每一层结束时向栈中压入NULL， 则遇到NULL就标志一层的结束，就可以存节点，分奇偶的选择性的反转当前层的节点。特别值得注意的是，循环到最后一行后，若是还压入NULL，会造成死循环，故在压人入NULL时要判断栈是否为空。这也是在

层次遍历为主题的基础上增加层反转条件即可。

/**
 * 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>> res;
        queue<TreeNode *> Q;
        if(root==NULL)  return res;  
        Q.push(root);
        Q.push(NULL);              //第一层的结束
        vector<int> levNode;       //存放每层的节点
        size_t level=1;            //第level层，初始为第一层
        while( !Q.empty())
        {
            TreeNode *cur=Q.front();
            Q.pop();
            if(cur)
            {
                levNode.push_back(cur->val);    //必须在if内，因栈顶节点可能为NULL
                if(cur->left)   
                    Q.push(cur->left);
                if(cur->right)
                    Q.push(cur->right);
            }
            else
            {
                if(level%2 ==0)        //偶数层，反转levNode再压入res
                    reverse(levNode.begin(),levNode.end());
                res.push_back(levNode);
                level++;

                levNode.clear();
                if( !Q.empty())   //最后一行，不要压入
                    Q.push(NULL);
            }
        }
        return res;
    }
};