DSA using JavaScript

Time Complexity: O(n) First pass processes the string in O(n). Second pass dequeues each element once, also O(n).
Space Complexity: O(n) The queue and hash map both store up to n elements.

Queue

function firstUniqCharBruteForce(s) {
  for (let i = 0; i < s.length; i++) {
    let count = 0;
    for (let j = 0; j < s.length; j++) {
      if (s[i] === s[j]) count++;
    }
    if (count === 1) return i;
  }
  return -1;
}

// Example Usage
console.log(firstUniqCharBruteForce("leetcode")); // Output: 0
console.log(firstUniqCharBruteForce("loveleetcode")); // Output: 2
console.log(firstUniqCharBruteForce("aabb")); // Output: -1

function firstUniqCharWithQueue(s) {
  const queue = []; // Queue to store characters with indices
  const charCount = {}; // Hash map to track frequencies

  // Populate the hash map and queue
  for (let i = 0; i < s.length; i++) {
    const char = s[i];
    charCount[char] = (charCount[char] || 0) + 1;
    queue.push([char, i]); // Store character and its index
  }

  // Process the queue to find the first unique character
  while (queue.length > 0) {
    const [char, index] = queue[0]; // Peek at the front of the queue
    if (charCount[char] === 1) {
      return index; // Found the first unique character
    }
    queue.shift(); // Dequeue if not unique
  }

  return -1; // No unique character found
}

// Example Usage
console.log(firstUniqCharWithQueue("leetcode")); // Output: 0
console.log(firstUniqCharWithQueue("loveleetcode")); // Output: 2
console.log(firstUniqCharWithQueue("aabb")); // Output: -1