以下代码实现了二叉树的哪种遍历方式

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def traverse(root):
    if not root:
        return
    print(root.val, end='')
    traverse(root.left)
    traverse(root.right)