Construct String from a Binary Tree (Leetcode 606)

Technique: FAITH technique of Recursion

Problem Link: https://leetcode.com/problems/construct-string-from-binary-tree/

class Solution:

    def tree2str(self, root: Optional[TreeNode]) -> str:
    
        def fun(root):
        
            if(root is None):
                return ""
            l=fun(root.left)
            r=fun(root.right)
            if(l is not "" and r is not ""):
                ans=str(root.val)+'('+l+')'+'('+r+')'
            elif(l is "" and r is not ""):
                ans=str(root.val)+'()'+'('+r+')'
            elif(l is not "" and r is ""):
                ans=str(root.val)+'('+l+')'
            else:
                ans=str(root.val)
            return ans
            
        return fun(root)