Binary Tree Level Order Traversal

Given the root of a binary tree, return the level order traversal of its nodes’ values. (i.e., from left to right, level by level).

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

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

        int i = 0;
        
        while(!q.empty()){   
            
            vector<int> temp;            
            int n = q.size();
            
            for(int i=0;i<n;i++){
                
                TreeNode* _node = q.front();
                q.pop();
                
                temp.push_back(_node->val);
                
                if(_node->left)
                    q.push(_node->left);
                
                if(_node->right)
                    q.push(_node->right);            
            }            
            ans.push_back(temp);
        }        
    
        return ans;
    }
};