Letter anagram grouping

lilfry@sjtu.edu.cn (lilfry) — Fri, 20 Jun 2025 00:00:00 +0800

Question description

You are given an array of strings, and you are asked to combine the anagrams together. The list of results can be returned in any order.

Example 1:

Input: strs = [“eat”, “tea”, “tan”, “ate”, “nat”, “bat”]

Output: [[“bat”],[“nat”,“tan”],[“ate”,“eat”,“tea”]]

explain:

There are no strings in strs that can be rearranged to form “bat”. The strings “nat” and “tan” are anagrams because they can be rearranged to form each other. The strings “ate” , “eat” and “tea” are anagrams because they can be rearranged to form each other. Example 2:

Find duplicate subtrees

lilfry@sjtu.edu.cn (lilfry) — Mon, 31 Mar 2025 21:32:46 +0800

Question description

Given the root node root of a binary tree, return all duplicate subtrees.

For duplicate subtrees of the same type, you only need to return the root node of any one of them.

If two trees have the same structure and the same node values, they are considered duplicates.

https://assets.leetcode.com/uploads/2020/08/16/e1.jpg

Input: root = [1,2,3,4,null,2,4,null,null,4] Output: [[2,4],[4]]

Problem-solving ideas

Use postorder traversal + hashing!

Idea:

Give each subtree an “ID card” (a string traversed in post-order)
Record the number of occurrences of each subtree (hash, counter)

Why does this method work?

String description uniquely determines the subtree structure For example, "2,4,#,#,#" can only correspond to one binary tree (root->left->right)

LeetCode on lilfry's library

Letter anagram grouping

Question description

Find duplicate subtrees

Question description

Problem-solving ideas

Use postorder traversal + hashing!

**Why does this method work? **

Why does this method work?