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].

Credits:
Special thanks to @elmirap for adding this problem and creating all test cases.

/**
 * 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> > result = new ArrayList<>();
        helper(root, result);
        return result;
    }
    
    private int helper(TreeNode root, List<List<Integer> > result) {
        if (root == null) return -1;
        int depth = 1 + Math.max(helper(root.left, result), helper(root.right, result));
        if (result.size() < depth + 1) result.add(new ArrayList<Integer>());
        result.get(depth).add(root.val);
        
        return depth;
    }
}