Queue DSA

class Queue { private int[] arr; private int front, rear, size, capacity; Queue(int capacity) { this.capacity = capacity; arr = new int[capacity]; front = 0; rear = -1; size = 0; } void enqueue(int data) { if (size == capacity) throw new RuntimeException("Queue full"); rear = (rear + 1) % capacity; arr[rear] = data; size++; } int dequeue() { if (isEmpty()) throw new RuntimeException("Queue empty"); int data = arr[front]; front = (front + 1) % capacity; size--; return data; } int peek() { return arr[front]; } boolean isEmpty() { return size == 0; } int size() { return size; } }

void bfs(int[][] graph, int start) { boolean[] visited = new boolean[graph.length]; Queue<Integer> queue = new LinkedList<>(); visited[start] = true; queue.offer(start); while (!queue.isEmpty()) { int node = queue.poll(); System.out.print(node + " "); for (int neighbor : graph[node]) { if (!visited[neighbor]) { visited[neighbor] = true; queue.offer(neighbor); } } } }

char[] firstNonRepeating(String stream) { int[] freq = new int[26]; Queue<Character> q = new LinkedList<>(); char[] result = new char[stream.length()]; for (int i = 0; i < stream.length(); i++) { char c = stream.charAt(i); freq[c - 'a']++; q.offer(c); while (!q.isEmpty() && freq[q.peek() - 'a'] > 1) q.poll(); result[i] = q.isEmpty() ? '#' : q.peek(); } return result; }