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]]

解答思路：
参见102. Binary Tree Level Order Traversal，将result翻转即可。

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