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]
]

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
    std::vector<std::vector<int> > levelOrderBottom(TreeNode *root) {
        dfs(root,0);
		std::reverse(res.begin(),res.end());
		return res;
    }
private:
	std::vector<std::vector<int>> res;
	void dfs(TreeNode *root, int level)
	{
		if(root == NULL) return;
		if(level == res.size())
		{
			res.push_back(std::vector<int>());
		}
		res[level].push_back(root->val);
		dfs(root->left,level+1);
		dfs(root->right,level+1);
	}
};