title: AttributeError: 'NoneType' object has no attribute 'left'
body: <p>Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.</p>
  
  <p>Note: A leaf is a node with no children.</p>
  
  <p>Example:</p>
  
  <p>Given the below binary tree and sum = 22,</p>
  
  <pre><code>      5
       / \
      4   8
     /   / \
    11  13  4
   /  \      \
  7    2      1
  </code></pre>
  
  <p>return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.</p>
  
  <pre><code>class Solution(object):
  def hasPathSum(self, root, sum):
      """
      :type root: TreeNode
      :type sum: int
      :rtype: bool
      """
      l = []
      def helper(tree, total):
          if not tree.left and not tree.right:
              l.append(total)
          else:
              helper(tree.left, total + tree.val)
              helper(tree.right, total + tree.val)
          return l
  
      if root:
          helper(root, 0)
          return sum in l
      return False
  </code></pre>
  
  <p>Can anyone tell me what's wrong with my code???</p>