def clean_semver(cls, version_spec):
        return re.sub(r"([<>=~])\s+?v?", "\\1", version_spec, re.IGNORECASE)
def dfs_print(root):
        if root is None:
            return
        dfs_print(root.left)
        print(str(root.key) + ' ', end='')
        dfs_print(root.right)