Strings

1. 1.Valid Anagram
   Frequency Arrayeasy
2. 2.Valid Palindrome
   Two Pointerseasy
3. 3.Longest Common Prefix
   Compare charseasy
4. 4.First Unique Character in a String
   Frequency Arrayeasy
5. 5.Reverse String
   Two Pointerseasy
6. 6.Reverse Words in a String
   Split + Reversemedium
7. 7.Longest Substring Without Repeating Characters
   Sliding Windowmedium
8. 8.Group Anagrams
   Sort / Freq keymedium
9. 9.Valid Parentheses
   Stackeasy
10. 10.String to Integer (atoi)
    Edge casesmedium
11. 11.Find the Index of the First Occurrence in a String
    Two Pointers / KMPeasy
12. 12.Longest Palindromic Substring
    Expand centermedium
13. 13.Palindromic Substrings
    Expand centermedium
14. 14.Minimum Window Substring
    Sliding Windowhard
15. 15.Decode String
    Stackmedium
16. 16.Multiply Strings
    Math simulationmedium
17. 17.Roman to Integer
    Map + scaneasy

1. 18.Isomorphic Strings
   Two Mapseasy
2. 19.Word Pattern
   Two Mapseasy
3. 20.Ransom Note
   Frequency Arrayeasy
4. 21.Add Strings
   Math carryeasy
5. 22.Compare Version Numbers
   Split + parsemedium
6. 23.Zigzag Conversion
   Row simulationmedium
7. 24.String Compression
   Two Pointersmedium
8. 25.Repeated Substring Pattern
   Double stringeasy
9. 26.Encode and Decode Strings
   Length prefixmedium
10. 27.Longest Repeating Character Replacement
    Sliding Windowmedium
11. 28.Permutation in String
    Sliding Windowmedium
12. 29.Find All Anagrams in a String
    Sliding Windowmedium
13. 30.Valid Palindrome II
    Two Pointerseasy

How to Think

1. Need character count?Hash Map / Frequency Array
2. Need compare two strings?Frequency / Two Pointers
3. Need longest substring?Sliding Window
4. Need substring without duplicate?Sliding Window + Hash Set/Map
5. Need smallest valid substring?Sliding Window
6. Need palindrome?Two Pointers / Expand from Center
7. Need group same characters?Sort string / Frequency signature
8. Need brackets validation?Stack
9. Need find pattern inside string?Two Pointers / KMP
10. Only lowercase a-z?[26]int
11. Need build result many times?strings.Builder (avoid concat)

Interview Rules

1. 1. Ask: case-sensitive or not?
2. 2. Ask: spaces/symbols matter?
3. 3. Check empty string.
4. 4. Check one character.
5. 5. Check repeated characters.
6. 6. Check uppercase/lowercase.
7. 7. Know: substring = continuous
8. 8. Know: subsequence = not necessarily continuous
9. 9. Try to reduce O(n²) substring checks using Sliding Window.
10. 10. In Go: strings are byte sequences; Unicode may need []rune.

💡 Strings are the 2nd most common interview topic. Master these 30 problems and you cover most string questions.