102. Binary Tree Level Order Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    public List<List<Integer>> levelOrder(TreeNode root) {
        
        if (root == null) return result;
        
        helper(root, 0);
        
        return result;
        
    }
    
    public void helper(TreeNode root, int level) {
        if (result.size() == level) {
            result.add(new ArrayList<Integer>());
        }
        
        result.get(level).add(root.val);
        
        if (root.left != null) {
            helper(root.left, level + 1);
        }
        
        if (root.right != null) {
            helper(root.right, level + 1);
        }
    }
}

Runtime: 0 ms, faster than 100.00% of Java online submissions for Binary Tree Level Order Traversal.
Memory Usage: 39.2 MB, less than 55.34% of Java online submissions for Binary Tree Level Order Traversal.

보자마자 냅다 재귀를~~

이거 나 중간고사에 나왔었음...ㅡㅡ