[LeetCode107]Binary Tree Level Order Traversal II 二叉树层次遍历

题目：

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

分类：Tree BFS 

代码：

递归：

 class Solution {
 protected:
     vector<vector<int>> ans;
     void dfs(TreeNode *root, int height){
         if (root == NULL)
             return;
         while (ans.size() <= height)
             ans.push_back(vector<int>());
         ans[height].push_back(root->val);
         dfs(root->left, height + );
         dfs(root->right, height + );
     }

 public:
     vector<vector<int>> levelOrderBottom(TreeNode* root) {
        dfs(root, );
        reverse(ans.begin(), ans.end());
         return ans;
     }
 };

非递归：

 /**
  * 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>> resultVec;
         if(!root)
             return resultVec;
         int parentSize = , childSize = ;
         int level = ;
         TreeNode *temp;
         queue<TreeNode*> nodes;
         nodes.push(root);
         resultVec.push_back(vector<int>());
         while(!nodes.empty())
         {
             temp  = nodes.front();
             resultVec[level].push_back(temp->val);
             nodes.pop();
             if(temp->left)
             {
                 nodes.push(temp->left);
                 childSize++;
             }
             if(temp->right)
             {
                 nodes.push(temp->right);
                 childSize++;
             }
             parentSize--;
             if(parentSize == )
             {
                 parentSize = childSize;
                 childSize = ;
                 level++;
                 resultVec.push_back(vector<int>());
             }
         }
         resultVec.erase(resultVec.end()-);
         reverse(resultVec.begin(),resultVec.end());
         return resultVec;
     }
 };