String on Jeanphilo Blog

LeetCode 49: Group Anagrams, from Scanning Groups to Building a Grouping Key

Sat, 10 Oct 2026 10:00:00 +0800

Build a Python solution one step at a time: compare two anagrams, scan group representatives, then derive sorted-string and character-count keys to understand why hash-table grouping works.

LeetCode 609: Find Duplicate File in System, from Parsing Records to Grouping by Content

Sat, 10 Oct 2026 10:00:00 +0800

Solve LeetCode 609 step by step: parse directory descriptions, build a correct pairwise baseline, group by content, and account for path construction and real-filesystem limitations.

LeetCode 139: Word Break, Starting from Prefix Reachability

Tue, 22 Sep 2026 12:00:00 +0800

Problem

Input and Output

Input: a string s and a list of strings wordDict
Every element in wordDict is an available word, and the same word may be reused
Output: return True if s can be split into one or more dictionary words; otherwise, return False
The split must cover the entire string without skipping or reordering characters

Examples

Input: s = "leetcode", wordDict = ["leet", "code"]
Output: True
Explanation: "leetcode" can be split into "leet" + "code"

Input: s = "applepenapple", wordDict = ["apple", "pen"]
Output: True
Explanation: "apple" + "pen" + "apple"; the same word may be reused

Input: s = "catsandog", wordDict = ["cats", "dog", "sand", "and", "cat"]
Output: False
Explanation: some prefixes can be formed, but the split cannot cover the entire string

Constraints

1 <= s.length <= 300
1 <= wordDict.length <= 1000
1 <= wordDict[i].length <= 20
s and wordDict[i] contain only lowercase English letters
All strings in wordDict are unique

Step 1: Make the Completed Prefix Visible

Start with one concrete question. For s = "leetcode", before deciding whether the whole string can be split, can we record how much of it has already been completed?

LeetCode 20: Valid Parentheses, Why Equal Counts Are Not Enough

Fri, 21 Aug 2026 00:00:00 +0000

Start from the counterexample ([)], separate equal bracket counts from correct types and closing order, and derive an O(n) stack solution.

LeetCode 394: Decode String by Saving and Restoring Nested Context

Fri, 21 Aug 2026 00:00:00 +0000

Start from the context lost by nested encoding, build a correct recursive parser, and make its saved frames explicit with a stack.

LeetCode 208: Implement Trie (Prefix Tree) in Python

Thu, 25 Jun 2026 13:58:00 +0800

Solve LeetCode 208 in Python by deriving Trie node fields, children traversal, is_end, insert, search, startsWith, and the exact interface LeetCode expects.

Trie Template Guide: Node Fields, Child Traversal, and Loop Invariants

Thu, 25 Jun 2026 13:58:00 +0800

Build a Python Trie template from first principles: derive children, is_end, insert/search traversal, prefix lookup, and the loop invariants behind each method.

Hot100: Palindrome Partitioning (Backtracking / Palindrome Table ACERS Guide)

Sun, 19 Apr 2026 14:49:56 +0800

A practical guide to LeetCode 131 that builds string-partition backtracking from scratch and shows why a palindrome table makes the search cleaner and more reusable.

LeetCode 1456: Maximum Number of Vowels in a Substring of Given Length (ACERS Guide)

Tue, 20 Jan 2026 13:40:45 +0800

Use a fixed-size sliding window to compute the maximum number of vowels in O(n). Includes engineering scenarios and multi-language implementations.