<p>Given the <code>root</code> of a binary tree, return <em>the bottom-up level order traversal of its nodes&#39; values</em>. (i.e., from left to right, level by level from leaf to root).</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg" style="width: 277px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> [[15,7],[9,20],[3]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1]
<strong>Output:</strong> [[1]]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> root = []
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 2000]</code>.</li>
	<li><code>-1000 &lt;= Node.val &lt;= 1000</code></li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        queue<pair<TreeNode*,int>> q;
        vector<vector<int>> ans;
        if(root == NULL){
            return ans;
        }
        stack<pair<int,int>> st;
        int h = -1;
        q.push({root,0});
        while(!q.empty()){
            TreeNode* node = q.front().first;
            int temp = node->val;
            int lebel = q.front().second;
            q.pop();

            if(lebel>h){
                h++;
                vector<int> v;
                ans.push_back(v);
            }

            ans[lebel].push_back(temp);
            if(node->left != NULL){
                q.push({node->left,lebel+1});
            }
            if(node->right != NULL){
                q.push({node->right,lebel+1});
            }
        }
        vector<vector<int>> main_ans;
        for(int i=ans.size()-1; i>=0; i--){
            main_ans.push_back(ans[i]);
        }
        return main_ans;
    }
};
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