[lintcode] problem 650 – find leaves of binary tree

public class  {
    public int val;
    public TreeNode left, right;
    public (int val) {
        this.val = val;
        this.left = this.right = null;
    }
}

public List<List<Integer>> findLeaves(TreeNode root) {
    Map<Integer, List<Integer>> map = new HashMap<>();
    List<List<Integer>> result = new ArrayList<>();

    findLeavesHelper(root, map);

    for (List<Integer> values : map.values())
        result.add(values);

    return result;
}

private int findLeavesHelper(TreeNode root, Map<Integer, List<Integer>> map) {
    if (root == null)
        return 0;

    int left = findLeavesHelper(root.left, map);
    int right = findLeavesHelper(root.right, map);

    int level = Math.max(left, right) + 1;
    map.putIfAbsent(level, new ArrayList<>());
    map.get(level).add(root.val);

    return level;
}