Binary Tree Level Order Traversal II

一、问题描述

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]]

二、思路

类似这一道题目：Binary Tree Level Order Traversal，只是增加了一行代码，将数组翻转即可。

三、代码

class Solution {public:    vector<vector<int> > ret;    void buildVector(TreeNode * root,int depth){        if(root == NULL) return;        if(ret.size() == depth)            ret.push_back(vector<int>());        ret[depth].push_back(root -> val);        buildVector(root -> left,depth + 1);        buildVector(root -> right,depth + 1);    }    vector<vector<int>> levelOrderBottom(TreeNode* root) {        buildVector(root,0);        reverse(ret.begin(),ret.end());        return ret;    }};