LeetCode 107. 二叉树的层次遍历 II（队列）

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回其自底向上的层次遍历为：

[
  [15,7],
  [9,20],
  [3]
]

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        if(root == NULL)
        	return {};
        queue<TreeNode*> q;
        vector<int> lv;
        vector<vector<int>> ans;
        q.push(root);
        int n;
        while(!q.empty())
        {
        	n = q.size();
            lv.clear();
        	while(n--)
        	{
        		root = q.front();
        		if(root->left)
        			q.push(root->left);
        		if(root->right)
        			q.push(root->right);
        		lv.push_back(root->val);
        		q.pop();
        	}
        	ans.push_back(lv);
        }
        reverse(ans.begin(),ans.end());
        return ans;
    }
};