Leetcode Binary Tree Level Order Traversal II

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

For example:
Given binary tree [3,9,20,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

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代码如下：

/** * Definition for a binary tree node. * 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>> result;         vector<vector<int>> tmp;        vector<int> data;          if(root == NULL)              return result;                    deque<TreeNode*> level;          level.push_back(root);          while(!level.empty())          {              data.clear();              int num = level.size();              for(int i=0;i<num;i++)              {                  if(level.front()->left != NULL)                      level.push_back(level.front()->left);                  if(level.front()->right != NULL)                      level.push_back(level.front()->right);                  data.push_back(level.front()->val);                  level.pop_front();              }              result.push_back(data);          }          for(int i=result.size()-1;i>=0;i--)            tmp.push_back(result[i]);        return tmp;      }};