Binary Tree Level Order Traversal II -- leetcode

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]]

 基本思路：

同问题 Binary Tree Level Order Traversal 一样，

在结果返回前，作一个reverse操作。

/** * 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> > ans;        dfs(ans, root, 0);        reverse(ans.begin(), ans.end());        return ans;    }        void dfs(vector<vector<int> > &ans, TreeNode *root, int level) {        if (!root) return;        if (level == ans.size())            ans.push_back(vector<int>());                ans[level].push_back(root->val);        dfs(ans, root->left, level+1);        dfs(ans, root->right, level+1);    }};