Top 50 String Problems
String manipulation problems test your ability to work with characters, substrings, and patterns. They are frequently asked in coding interviews.
Essential String Problems:
- Longest Substring Without Repeating Characters
- Longest Palindromic Substring
- Valid Palindrome
- Group Anagrams
- Valid Anagram
- Longest Common Prefix
- String to Integer (atoi)
- Implement strStr() (substring search)
- Reverse String
- Reverse Words in a String
- Zigzag Conversion
- Roman to Integer / Integer to Roman
- Count and Say
- Multiply Strings
- Longest Repeating Character Replacement
- Minimum Window Substring
- Encode and Decode Strings
- Valid Parentheses (string version)
- Generate Parentheses
- Word Break
Strategies:
- Sliding window for substring constraints.
- HashMap for frequency counting (anagrams).
- Two pointers for palindrome checks.
- Dynamic programming for longest palindromic substring, edit distance.
- Trie for prefix search.
Example: Longest Substring Without Repeating Characters
public static int lengthOfLongestSubstring(String s) {
Set set = new HashSet<>();
int left = 0, maxLen = 0;
for (int right = 0; right < s.length(); right++) {
while (set.contains(s.charAt(right))) {
set.remove(s.charAt(left));
left++;
}
set.add(s.charAt(right));
maxLen = Math.max(maxLen, right - left + 1);
}
return maxLen;
}
Two Minute Drill
- String problems require familiarity with character arrays, StringBuilder, and algorithms like KMP (though rarely needed).
- Sliding window and hash maps are your best friends.
- Practice palindromes, anagrams, and parsing.
Need more clarification?
Drop us an email at career@quipoinfotech.com