107. 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]
]

思路：跟Binary Tree Level Order Traversal 一样，只是写个反过来的程序而已。

/** * 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>> result;        vector<vector<int>> result_final;    vector<vector<int>> levelOrderBottom(TreeNode* root) {        if(root==NULL)  return result;        order(root,0);        for(int i=result.size()-1;i>=0;i--)        {            result_final.push_back(result[i]);        }        return result_final;    }    void order(TreeNode* root,int level)    {        if(root==NULL) return ;        if(result.size()<level+1)        {            result.push_back(vector<int> {});        }        result[level].push_back(root->val);        if(root->left) order(root->left,level+1);        if(root->right) order(root->right,level+1);    }};