107. Binary Tree Level Order Traversal II

/**
 * 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) {
        vector<vector<int>> ans;
        if(root == NULL)
            return ans;
        
        queue<TreeNode*> q;
        q.push(root);

        while(!q.empty())
        {
            int size = q.size();
            vector<int> temp;

            for(int i=0; i<size; i++)
            {
                TreeNode* front = q.front();
                q.pop();

                temp.push_back(front -> val);
                if(front -> left)
                    q.push(front -> left);
                if(front -> right)
                    q.push(front -> right);
            }

            ans.push_back(temp);
        }
        reverse(ans.begin(), ans.end());
        
        return ans;
    }
};