Queue data structure, deque, and circular queue problems.

• FIFO (First In First Out) principle
• Queue implementation
• Circular queue
• Deque (double-ended queue)
• Sliding window maximum
• Queue using stacks
• BFS applications

• Use std::queue for basic FIFO operations
• Use std::deque for double-ended operations
• Queue is natural for BFS traversal
• Sliding window problems often use deque
• Consider monotonic deque for optimization