[LeetCode] 652. Find Duplicate Subtrees

 Problem : https://leetcode.com/problems/find-duplicate-subtrees/

A tree can be represented by its path. 2 trees are identical if their dumped paths are the same.

Use postorder traversal to find sub-trees with same dumped path.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def findDuplicateSubtrees(self, root: Optional[TreeNode]) -> List[Optional[TreeNode]]:
        
        seen = defaultdict(list)
        result = []
        
        def postorder(node):
            if not node: return "#"
            
            key = '(' + str(node.val) + ')'+ postorder(node.left) + postorder(node.right)
            seen[key].append(node)
            
            if len(seen[key]) == 2:
                result.append(node)
            
            return key
        
        postorder(root)
        
        return result