以下函数createTree()构造的树是什么类型？

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def create_tree():
    root = TreeNode(1)
    root.left = TreeNode(2)
    root.right = TreeNode(3)
    root.left.left = TreeNode(4)
    root.left.right = TreeNode(5)
    return root