Leetcode: Find Leaves of Binary Tree

Given a binary tree, collect a tree's nodes as if you were doing this: Collect and remove all leaves, repeat until the tree is empty.

Example:
Given binary tree 
          1
         / 
        2   3
       /      
      4   5    
Returns [4, 5, 3], [2], [1].

Explanation:
1. Removing the leaves [4, 5, 3] would result in this tree:

          1
         / 
        2          
2. Now removing the leaf [2] would result in this tree:

          1          
3. Now removing the leaf [1] would result in the empty tree:

          []         
Returns [4, 5, 3], [2], [1].

Better Solution: https://discuss.leetcode.com/topic/49194/10-lines-simple-java-solution-using-recursion-with-explanation/2

For this question we need to take bottom-up approach. The key is to find the height of each node. The height of a node is the number of edges from the node to the deepest leaf.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> findLeaves(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        helper(root, res);
        return res;
    }
    
    public int helper(TreeNode cur, List<List<Integer>> res) {
        if (cur == null) return -1;
        int level = 1 + Math.max(helper(cur.left, res), helper(cur.right, res));
        if (res.size() <= level) 
            res.add(new ArrayList<Integer>());
        res.get(level).add(cur.val);
        cur.left = cur.right = null;
        return level;
    }
}

First time solution: HashSet+ DFS

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> findLeaves(TreeNode root) {
        ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
        if (root == null) return res;
        HashSet<TreeNode> visited = new HashSet<>();
        while (!visited.contains(root)) {
            ArrayList<Integer> leaves = new ArrayList<Integer>();
            helper(root, leaves, visited);
            res.add(new ArrayList<Integer>(leaves));
        }
        return res;
    }
    
    public void helper(TreeNode cur, ArrayList<Integer> leaves, HashSet<TreeNode> visited) {
        if ((cur.left==null || visited.contains(cur.left)) && (cur.right==null || visited.contains(cur.right))) {
            leaves.add(cur.val);
            visited.add(cur);
            return;
        }
        if (cur.left!=null && !visited.contains(cur.left))
            helper(cur.left, leaves, visited);
        if (cur.right!=null && !visited.contains(cur.right))
            helper(cur.right, leaves, visited);
    }
}