Leetcode: Binary Tree Level Order Traversal

    3
   / \
  9  20
    /  \
   15   7

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

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution
{
private:
    void visit(TreeNode *node, int depth, vector<vector<int>> &result)
    {
    	if (node == NULL)
    	{
    		return;
    	}
    
    	if (result.size() > depth)
    	{
    		result[depth].push_back(node->val);
    	}
    	else
    	{
    		vector<int> level;
    		level.push_back(node->val);
    		result.push_back(level);
    	}
    
    	visit(node->left, depth + 1, result);
    	visit(node->right, depth + 1, result);
    }
public:
    vector<vector<int>> levelOrder(TreeNode *root)
    {
    	vector<vector<int>> result;
    	visit(root, 0, result);
    	return result;
    }
};