LeetCode: 107_Binary Tree Level Order Traversal II | 二叉树自底向上的层次遍历 | Easy

本题和上题一样同属于层次遍历，不同的是本题从底层往上遍历，如下：

代码如下：

 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> > levelOrderTraversalII(TreeNode *root) //非递归的中序遍历(用栈实现)
 {

     queue<TreeNode *> tree_queue;
     vector<vector<int> > tree_vector, tree_rvector;
     vector<int> svector;
     int nCount = ;

     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);
             nCount ++;
             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);
         }
     }
     while (nCount > ){
         svector = tree_vector.at(nCount - );
         tree_rvector.push_back(svector);
         nCount --;
     }
     return tree_rvector;
 }