Interview Q&A / Data Structures & Algorithms

1. How do you check if a string of brackets is valid (balanced parentheses)?

   Scan the string with a stack. Push every opening bracket. For every closing bracket, the stack must be non-empty and its top must be the matching opening bracket, which you then pop. The string is valid only if the stack is empty at the end. O(n) time, O(n) space.

   BeginnerIn-depthCoding3 min read
2. How do you check if a string is a palindrome, ignoring spaces, punctuation and case?

   Use two pointers from both ends. Skip characters that are not letters or digits, compare the rest in lowercase, and move inward. If any pair differs, it is not a palindrome.

   BeginnerQuick answerCoding1 min read
3. How do you check if one string is a rotation of another?

   They must have the same length, and the second string must appear inside the first string written twice. For example, "waterbottle" doubled contains "erbottlewat".

   BeginnerQuick answerCoding1 min read
4. How do you check whether two strings are anagrams of each other?

   Two strings are anagrams if they have the same letters with the same counts. Count the characters of the first string, subtract using the second, and check that every count ends at zero.

   BeginnerQuick answerCoding1 min read
5. How do you find the first non-repeating character in a string?

   Count how often each character appears in one pass, then scan again and return the first character whose count is 1.

   BeginnerQuick answerCoding1 min read
6. How do you find the longest common prefix among an array of strings?

   Take the first string as the candidate prefix and shorten it until every other string starts with it. Return what is left, which may be an empty string.

   BeginnerQuick answerCoding1 min read
7. How do you reverse the order of words in a sentence, ignoring extra spaces?

   Trim the string, split on one or more whitespace characters, reverse the array of words, and join with a single space.

   BeginnerQuick answerCoding1 min read