Construct String from Binary Tree — Python

# Time:  O(n)
# Space: O(h)

class Solution(object):
    def tree2str(self, t):
        """
        :type t: TreeNode
        :rtype: str
        """
        if not t: return ""
        s = str(t.val)
        if t.left or t.right:
            s += "(" + self.tree2str(t.left) + ")"
        if t.right:
            s += "(" + self.tree2str(t.right) + ")"
        return s