107. Binary Tree Level Order Traversal II

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]]

解题思路：

顾名思义，二叉树的层次遍历的逆序输出。

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