606. Construct String from Binary Tree

"""
这道题只需要注意一点就是，当左子树为null而右子树不为null的时候要把左子树'()'打印出来
"""
class Solution:
    def preorder(self, root):
        if not root:
            return ''

        if not root.left and root.right:
            return "(" + str(root.val) + "()" + self.preorder(root.right) + ")"
        else:
            return "(" + str(root.val) + self.preorder(root.left) + self.preorder(root.right) + ")"

    def tree2str(self, t):
        """
        :type t: TreeNode
        :rtype: str
        """
        _str = self.preorder(t)

        return _str[1:-1]