🔤 Strings

Strings are arrays of characters with a few extra twists: immutability, character sets, and pattern matching. Most string problems reduce to sliding window, hashing, or two pointers.

This category contains 71 problems. Use the patterns below to recognize what's being asked, then jump to the problem list at the bottom.

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🧠 Key Patterns

• Sliding Window + Frequency Map — Longest/shortest substring with constraint on distinct chars or anagram windows.
• Two Pointers from Ends — Palindrome checks, reversing in place.
• Hashing & Frequency Counts — Anagram detection, group-by-signature.
• Expand Around Center — All palindromic substrings in $O(n^2)$.
• KMP / Z-Algorithm / Rolling Hash — Pattern matching in $O(n + m)$.
• Stack — Balanced parens, decode strings, simplify path.

⚠️ Common Pitfalls

• C#: avoid string += in a loop — use StringBuilder.
• Char range assumption (a..z vs full Unicode). Clarify before coding.
• Off-by-one when comparing two pointers i < j in palindrome expansion.

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📚 Study Resources

📺 Videos

• NeetCode — Sliding Window playlist
• Aditya Verma — String DP

📖 Books

• Cracking the Coding Interview — Ch. 1 (Arrays & Strings)
• Algorithms — Sedgewick & Wayne — Ch. 5 (Strings)

🌐 Articles & References

• CP-Algorithms — String processing
• LeetCode — String Patterns

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💻 All Strings Problems

📄️Add Strings

LeetCode 415 | Difficulty: Easy

📄️Buddy Strings

LeetCode 889 | Difficulty: Easy

📄️Check if One String Swap Can Make Strings Equal

LeetCode 1915 | Difficulty: Easy

📄️Compare Version Numbers

LeetCode 165 | Difficulty: Medium

📄️Count Binary Substrings

LeetCode 696 | Difficulty: Easy

📄️Count Number of Homogenous Substrings

LeetCode 1885 | Difficulty: Medium

📄️Decode String

LeetCode 394 | Difficulty: Medium

📄️Determine if Two Strings Are Close

LeetCode 1777 | Difficulty: Medium

📄️DI String Match

LeetCode 979 | Difficulty: Easy

📄️Encode and Decode Strings

LeetCode Link

📄️Evaluate Reverse Polish Notation

LeetCode 150 | Difficulty: Medium

📄️Find First Palindromic String in the Array

LeetCode 2231 | Difficulty: Easy

📄️Find Unique Binary String

LeetCode 2107 | Difficulty: Medium

📄️First Unique Character in a String

LeetCode 387 | Difficulty: Easy

📄️Group Anagrams

LeetCode 49 | Difficulty: Medium

📄️Group Shifted Strings

LeetCode Link

📄️Implement strStr()

LeetCode Link

📄️Integer to English Words

LeetCode 273 | Difficulty: Hard

📄️Isomorphic Strings

LeetCode 205 | Difficulty: Easy

📄️Length of Last Word

LeetCode 58 | Difficulty: Easy

📄️Longest Common Prefix

LeetCode 14 | Difficulty: Easy

📄️Longest Happy String

LeetCode 1304 | Difficulty: Medium

📄️Longest Palindrome

LeetCode 409 | Difficulty: Easy

📄️Longest Palindromic Substring

LeetCode 5 | Difficulty: Medium

📄️Longest Substring with At Least K Repeating Characters

LeetCode 395 | Difficulty: Medium

📄️Longest Substring with At Most K Distinct Characters

LeetCode Link

📄️Longest Substring with At Most Two Distinct Characters

LeetCode Link

📄️Longest Substring Without Repeating Characters

LeetCode 3 | Difficulty: Medium

📄️Longest Valid Parentheses

LeetCode 32 | Difficulty: Hard

📄️Maximum Length of a Concatenated String with Unique Characters

LeetCode 1360 | Difficulty: Medium

📄️Minimum Add to Make Parentheses Valid

LeetCode 957 | Difficulty: Medium

📄️Minimum Insertion Steps to Make a String Palindrome

LeetCode 1437 | Difficulty: Hard

📄️Minimum Insertions to Balance a Parentheses String

LeetCode 1648 | Difficulty: Medium

📄️Minimum Number of Swaps to Make the String Balanced

LeetCode 2095 | Difficulty: Medium

📄️Minimum Window Substring

LeetCode 76 | Difficulty: Hard

📄️Most Common Word

LeetCode 837 | Difficulty: Easy

📄️Multiply Strings

LeetCode 43 | Difficulty: Medium

📄️Next Palindrome Using Same Digits

LeetCode Link

📄️Number of Steps to Reduce a Number to Zero

LeetCode 1444 | Difficulty: Easy

📄️Number of Substrings Containing All Three Characters

LeetCode 1460 | Difficulty: Medium

📄️Number of Substrings With Only 1s

LeetCode 1636 | Difficulty: Medium

📄️Palindrome Number

LeetCode 9 | Difficulty: Easy

📄️Palindrome Permutation

LeetCode Link

📄️Palindromic Substrings

LeetCode 647 | Difficulty: Medium

📄️Permutations

Permutations of a String

📄️Ransom Note

LeetCode 383 | Difficulty: Easy

📄️Reorganize String

LeetCode 778 | Difficulty: Medium

📄️Repeated Substring Pattern

LeetCode 459 | Difficulty: Easy

📄️Reverse Nodes in k-Group

LeetCode 25 | Difficulty: Hard

📄️Reverse String

LeetCode 344 | Difficulty: Easy

📄️Reverse String II

LeetCode 541 | Difficulty: Easy

📄️Reverse Vowels of a String

LeetCode 345 | Difficulty: Easy

📄️Reverse Words in a String

LeetCode 151 | Difficulty: Medium

📄️Reverse Words in a String II

LeetCode Link

📄️Reverse Words in a String III

LeetCode 557 | Difficulty: Easy

📄️Rotate String

LeetCode 812 | Difficulty: Easy

📄️Simplify Path

LeetCode 71 | Difficulty: Medium

📄️Smallest String Starting From Leaf

LeetCode 1030 | Difficulty: Medium

📄️String Compression

LeetCode 443 | Difficulty: Medium

📄️String to Integer (atoi)

LeetCode 8 | Difficulty: Medium

📄️String Without AAA or BBB

LeetCode 1026 | Difficulty: Medium

📄️Substring with Concatenation of All Words

LeetCode 30 | Difficulty: Hard

📄️Substrings That Begin and End With the Same Letter

LeetCode Link

📄️Text Justification

LeetCode 68 | Difficulty: Hard

📄️Valid Anagram

LeetCode 242 | Difficulty: Easy

📄️Valid Number

LeetCode 65 | Difficulty: Hard

📄️Valid Palindrome

LeetCode 125 | Difficulty: Easy

📄️Valid Palindrome II

LeetCode 680 | Difficulty: Easy

📄️Valid Parentheses

LeetCode 20 | Difficulty: Easy

📄️Valid Parenthesis String

LeetCode 678 | Difficulty: Medium

📄️Zigzag Conversion

LeetCode 6 | Difficulty: Medium