String Interview Problems

Picture this. 🎯 You're sitting in a coding interview. The interviewer smiles and says:

"Can you write a function to check if a string has valid brackets?"

Your heart beats a little faster. 😅 But then you remember: string problems follow patterns. Once you know the patterns, most questions become familiar friends.

In this lesson, you'll practise 16 popular interview questions on strings, from warm-ups to the ones that big companies love to ask.

How to Use This Lesson

For every problem:

1. 📖 Read the problem and the examples.
2. 🤔 Think about how you'd solve it by hand first.
3. ⌨️ Try it yourself in the playground.
4. ✅ Then read the idea and the solution.

In a real interview, talk through your thinking out loud. Interviewers care about how you think, not just the final code. 🗣️

The 6 Patterns You'll Use Again and Again 🧠

Almost every string problem uses one (or more) of these:

A quick word on speed (Big-O) ⏱️

Interviewers often ask, "How fast is your solution?" We describe this with Big-O, where n is the length of the string:

Aim for O(n) when you can, but a working O(n²) solution is always better than no solution!

🟢 Warm-Up Round

Problem 1: Reverse Words in a Sentence

Task: Reverse the order of the words, not the letters. Remove extra spaces.

Examples:

• "I love JavaScript" → "JavaScript love I"
• " hello world " → "world hello"

💡 Idea: Split → change → join. Trim the ends, split on one or more spaces, reverse the array, and join with a single space.

⏱️ Time: O(n)

Follow-up the interviewer might ask: "Reverse the letters of each word, but keep the word order."

Problem 2: Valid Anagram

Task: Return true if two strings contain exactly the same letters, in any order.

Examples:

• "listen", "silent" → true
• "rat", "car" → false

💡 Idea: Frequency map. Count every letter in the first word, then subtract for every letter in the second. If everything comes back to zero, they're anagrams.

⏱️ Time: O(n)

💡 A shorter answer is to sort both strings and compare them, but that's O(n log n). Mention both in an interview. It shows you know the trade-offs!

Problem 3: Longest Common Prefix

Task: Find the longest starting part that all the words share.

Examples:

• ["flower", "flow", "flight"] → "fl"
• ["dog", "car", "race"] → ""

💡 Idea: Start by assuming the whole first word is the prefix. For each other word, keep cutting the prefix shorter until the word starts with it.

Problem 4: Check if One String Is a Rotation of Another

Task: Is b a rotation of a? A rotation moves some characters from the front to the back.

Examples:

• "waterbottle", "erbottlewat" → true
• "hello", "lohel" → true
• "hello", "olleh" → false

💡 Idea: Here's a clever trick 🪄. If you join a with itself, every possible rotation appears inside it!

a + a = "hellohello"
          ↑↑↑↑↑
          "lohel" is inside ✅

🟡 Medium Round

Problem 5: Valid Palindrome (Ignoring Symbols)

Task: Check if a sentence reads the same forwards and backwards, ignoring case, spaces, and punctuation.

Examples:

• "A man, a plan, a canal: Panama" → true
• "race a car" → false

💡 Idea: Two pointers. One starts at the left, one at the right. Skip anything that isn't a letter or number. Compare the characters and move inward. It's like two friends walking towards each other and checking that they're holding the same card. 🃏

⏱️ Time: O(n). It also uses no extra copy of the string, which interviewers like!

Problem 6: Valid Parentheses ⭐

Task: Check if every bracket ( ) { } [ ] is closed in the correct order.

Examples:

• "()[]{}" → true
• "{[()]}" → true
• "(]" → false
• "([)]" → false

💡 Idea: Use a stack 🍽️, like a pile of plates. Every opening bracket goes on top of the pile. When you see a closing bracket, the top plate must match. If it doesn't, it's invalid. At the end, the pile must be empty.

"{[()]}"
 {        push  → [ { ]
  [       push  → [ {, [ ]
   (      push  → [ {, [, ( ]
    )     pop ( ✅ → [ {, [ ]
     ]    pop [ ✅ → [ { ]
      }   pop { ✅ → [ ]   empty → valid!

⏱️ Time: O(n)

Problem 7: Group Anagrams Together

Task: Group words that are anagrams of each other.

Example: ["eat", "tea", "tan", "ate", "nat", "bat"] → [["eat", "tea", "ate"], ["tan", "nat"], ["bat"]]

💡 Idea: Anagrams look identical when their letters are sorted: "eat", "tea", and "ate" all become "aet". Use that sorted version as a key in an object, and collect the words under it.

Problem 8: First Non-Repeating Character

Task: Return the index of the first character that appears only once, or -1 if there isn't one.

Examples:

• "leetcode" → 0 ("l")
• "loveleetcode" → 2 ("v")
• "aabb" → -1

💡 Idea: Frequency map, two passes. First, count everything. Then walk again and return the first character with a count of 1.

Problem 9: String Compression

Task: Compress repeated characters as character + count. If the result isn't shorter, return the original.

Examples:

• "aabcccccaaa" → "a2b1c5a3"
• "abc" → "abc" (compressed "a1b1c1" is longer)

💡 Idea: Walk through the string. Count how many times the same character repeats in a row. When it changes, save the character and its count. Build the result in an array and join once at the end, because strings are immutable.

Problem 10: Isomorphic Strings

Task: Two strings are isomorphic if the letters of one can be replaced to get the other, with each letter always mapping to the same letter.

Examples:

• "egg", "add" → true (e→a, g→d)
• "foo", "bar" → false (o can't be both a and r)
• "paper", "title" → true

💡 Idea: Keep two maps: one from s to t, and one from t to s. If any letter tries to map to two different letters, it's not isomorphic. It's like a secret code 🔐: each letter must always have the same code letter.

Problem 11: Roman Numerals to Numbers 🏛️

Task: Convert a Roman numeral to a normal number.

Examples:

• "III" → 3
• "IX" → 9
• "MCMXCIV" → 1994

💡 Idea: Usually you add each value. But when a smaller value comes before a bigger one (like I before X in IX), you subtract it instead.

Problem 12: Word Pattern

Task: Check if a sentence follows a pattern, where each letter matches exactly one word.

Examples:

• pattern "abba", text "dog cat cat dog" → true
• pattern "abba", text "dog cat cat fish" → false
• pattern "aaaa", text "dog cat cat dog" → false

💡 Idea: This is Problem 10 in disguise! 🎭 Instead of mapping letter → letter, map letter → word.

🧠 Interview tip: Spotting that a new problem is an old problem in disguise is a superpower. Always ask yourself, "Have I seen something like this before?"

🔴 Interview Favourites

Problem 13: Longest Substring Without Repeating Characters ⭐⭐

This is one of the most asked string questions of all time!

Task: Find the length of the longest continuous part of the string with no repeated characters.

Examples:

• "abcabcbb" → 3 ("abc")
• "bbbbb" → 1 ("b")
• "pwwkew" → 3 ("wke")

💡 Idea: Sliding window 🪟. Imagine a window sliding over the string.

• Move the right edge forward, one character at a time.
• If the new character is already inside the window, move the left edge just past its earlier position.
• Track the biggest window you've seen.

Think of a train compartment 🚃 where no two passengers can have the same name. When someone new boards with a name that's already inside, people at the front get off until the old one with that name has left.

"abcabcbb"
 [abc]abcbb     window "abc" → size 3
  [bca]bcbb     "a" repeated → left moves → size 3
   [cab]cbb     "b" repeated → left moves → size 3
   ...          best stays 3

⏱️ Time: O(n). Each character is visited only once. 🚀

Problem 14: Longest Palindromic Substring ⭐⭐

Task: Find the longest part of the string that is a palindrome.

Examples:

• "babad" → "bab" (or "aba")
• "cbbd" → "bb"
• "forgeeksskeegfor" → "geeksskeeg"

💡 Idea: Expand around the centre. Every palindrome has a middle. Try every position as a middle and grow outward while both sides match, like ripples 🌊 spreading out from a stone dropped in water.

There are two kinds of middles:

• Odd length, one centre character: "aba" → centre b
• Even length, the centre is between two characters: "abba" → centre between b and b

⏱️ Time: O(n²), which is the expected answer in most interviews.

Problem 15: Count and Say 🗣️

Task: Each term describes the previous one out loud.

1          → "one 1"          → 11
11         → "two 1s"         → 21
21         → "one 2, one 1"   → 1211
1211       → "one 1, one 2, two 1s" → 111221

Find the nth term.

💡 Idea: It's string compression (Problem 9) again, just with the count first and the character second! Repeat it n - 1 times.

Problem 16: Minimum Window Substring ⭐⭐⭐ (Challenge!)

This is a hard one, often asked at top companies. Don't worry if it takes a few tries! 💪

Task: Find the smallest part of s that contains all the characters of t (including duplicates).

Examples:

• s = "ADOBECODEBANC", t = "ABC" → "BANC"
• s = "a", t = "aa" → "" (not enough as)

💡 Idea: Sliding window again, but smarter.

1. Move the right edge to grow the window until it contains everything in t.
2. Then move the left edge to shrink it as small as possible, while it still contains everything.
3. Record the smallest valid window. Repeat.

It's like packing a school bag 🎒: first grab things until you have everything on the list, then take out anything extra you don't need.

⏱️ Time: O(n). Each character enters and leaves the window at most once.

🎯 Practice Challenges (Try Yourself!)

No solutions this time. You've got this! 💪

1. Reverse vowels only: "hello" → "holle" (Hint: two pointers)
2. Most frequent character: "programming" → "g" (or "r" / "m" on a tie)
3. Check if a string has all unique characters, without using a Set.
4. Longest word made of letters only: ignore numbers and symbols.
5. Valid palindrome II: Can the string become a palindrome by removing at most one character? "abca" → true
6. Integer to Roman: 1994 → "MCMXCIV" (the reverse of Problem 11)
7. Find all anagrams: Find every starting index in "cbaebabacd" where an anagram of "abc" begins → [0, 6] (Hint: sliding window of fixed size)

🗣️ Interview Tips for String Questions

• Ask questions first. "Is it case-sensitive? Can there be spaces? What about an empty string?" Interviewers love this.
• Start simple. Give a working brute-force answer, then say "Now let me make it faster."
• Test edge cases out loud: empty string "", one character "a", all the same "aaaa", and spaces or symbols.
• Remember that strings are immutable. Building a big result with += in a loop creates many new strings. Collecting pieces in an array and using join() once is a nice touch.
• Name your pattern. Saying "I'll use a sliding window here" shows real experience. 🏆

Pattern Cheat Sheet

Key Takeaways

• Most string interview questions use a small set of patterns: frequency maps, two pointers, sliding windows, stacks, and expanding around the centre.
• Frequency maps solve anagram and uniqueness problems in O(n), and a stack is the go-to tool for matching brackets.
• The sliding window pattern turns "longest/shortest substring" problems from O(n²) into O(n).
• Many "new" problems are old ones in disguise, like Word Pattern (isomorphic strings) and Count and Say (compression).
• In interviews, think out loud, start simple, test edge cases, and then improve your solution. 🚀