/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> findLeaves(TreeNode root) {
        List<List<Integer>> result = new LinkedList<>();
        
        if (root == null) return result;
                
        while (root != null) {
            ArrayList<Integer> res = new ArrayList<>();
            root = findLeaves(root, res);
            result.add(res);
        }
        
        return result;
    }
    
    private TreeNode findLeaves(TreeNode root, List<Integer> result) {
        if (root == null) return null;
        
        // add the leave node to the result
        if (root.left == null && root.right == null) {
            result.add(root.val);
            return null; // indicate the parent, for leave node & stop
        }
        
        if (root.left != null)
            root.left = findLeaves(root.left, result); 
        
        if (root.right != null) 
            root.right = findLeaves(root.right, result); 
        
        return root;
    }
}