LeetCode: 102_Binary Tree Level Order Traversal | 二叉树自顶向下的层次遍历 | Easy

题目：Binay Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

 For example:
 Given binary tree {,,,#,#,,},

如下一棵树

转换之后需要输出这样的形式：

如下，见代码：

 struct TreeNode {
     int        val;
     TreeNode*    left;
     TreeNode*    right;
     TreeNode():val(),left(NULL),right(NULL) {}
     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 };

 vector<vector<int> > levelOrderTraversal(TreeNode *root) //非递归的中序遍历(用栈实现)
 {

     queue<TreeNode *> tree_queue;
     vector<vector<int> > tree_vector;
     vector<int> svector;

     if (NULL == root) {
         return tree_vector;
     }
     TreeNode *pTemp = root;
     tree_queue.push(root);
     tree_queue.push(NULL); //the end of one level.

     while (true) {
         TreeNode *pTemp = tree_queue.front();
         tree_queue.pop();

         if (!pTemp) { //get the null, put vector<> to vector<vector<>>
             tree_vector.push_back(svector);
             svector.clear();
             if (tree_queue.empty())
                 break;
             tree_queue.push(NULL);
         }
         else {
             svector.push_back(pTemp->val);
             if (pTemp->left)
                 tree_queue.push(pTemp->left);
             if (pTemp->right)
                 tree_queue.push(pTemp->right);
         }
     }
     return tree_vector;
 }