Unique Characters - FreeCodeCamp #30 Daily Challenge

Problem Statement

Given a string, determine if all characters are unique (no repetitions).

⚠️ Note: Uppercase and lowercase letters are considered different (“a” ≠ “A”).

Initial Analysis

What are we looking for?

Check if a string has all distinct characters.

Test Cases

Solution Development

Chosen Strategy

We use a Set to store the characters we’ve already seen. If we find a repeated character, we instantly return false. If we reach the end with no repetitions, we return true.

JavaScript Code

// Returns true if all characters are unique
function hasUniqueCharacters(str) {
  const seen = new Set()
  for (const char of str) {
    if (seen.has(char)) {
      // Found a duplicate
      return false
    }
    seen.add(char)
  }
  // All characters are unique
  return true
}

// Usage examples
console.log(hasUniqueCharacters('abc')) // true
console.log(hasUniqueCharacters('hello')) // false
console.log(hasUniqueCharacters('aA')) // true

Complexity Analysis

• Time: $O(n)$, where $n$ is the length of the string (we traverse each character once).
• Space: $O(n)$ in the worst case (all characters are unique and stored in the Set).

Edge Cases and Considerations

• Empty string: returns true (no repetitions).
• Special characters, numbers, and letters: all count as different.
• Uppercase and lowercase: considered different (“a” ≠ “A”).
• Long strings with all unique characters: higher memory usage.

Reflections and Learnings

What did we learn?

• Set is ideal for efficiently detecting duplicates.
• Early detection of repetitions saves time.
• This pattern is useful in validations, passwords, and more.

Resources and References

• Set in JavaScript (MDN)