35-First-Non-Repeating-Character-in-Stream.md

First Non-Repeating Character in Stream

Problem Link

Practice Problem

---

Pattern

• Queue
• Hash Table

---

Approach

Maintain character frequencies and a queue of candidates, removing repeated characters from the front.

---

Time Complexity

O(n)

Space Complexity

O(1)

---

Java Solution

import java.util.*;
class FirstNonRepeatingStream {
    private int[] count = new int[26];
    private Queue<Character> queue = new LinkedList<>();

    public char add(char c) {
        count[c - 'a']++;
        queue.offer(c);
        while (!queue.isEmpty() && count[queue.peek() - 'a'] > 1) queue.poll();
        return queue.isEmpty() ? '#' : queue.peek();
    }
}