LeetCode 366 Find Leaves of Binary Tree

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Solution(object):
    # def findLeaves(self, root):
    #     """
    #     :type root: TreeNode
    #     :rtype: List[List[int]]
    #     """
    #     res = []
    #     if root is None:
    #         return []
    #     stack = [root]
    #     check_set = set()
    #     while len(stack) > 0:
    #         curr = stack.pop()
    #         check_set.add(curr)
    #         if curr.left:
    #             stack.append(curr.left)
    #         if curr.right:
    #             stack.append(curr.right)
    #     while len(check_set) > 0:
    #         curr = []
    #         for node in check_set:
    #             if (node.left is None or node.left not in check_set) and\
    #                (node.right is None or node.right not in check_set):
    #                 curr.append(node)
    #         res.append([node.val for node in curr])
    #         for node in curr:
    #             check_set.remove(node)
    #     return res

    def findLeaves(self, root):
        res = []
        self.findLeaves_helper(root, res)
        return res

    def findLeaves_helper(self, node, res):
        if node is None:
            return -1
        level = 1 + max(self.findLeaves_helper(node.left, res), self.findLeaves_helper(node.right, res))
        if len(res) < level + 1:
            res.append([])
        res[level].append(node.val)
        return level