Stacks and Queues

Announcements USACO Open competition results are out o Congrats to Johnny for scoring 2nd in the US USACO Finalists are also announced o Lynbrook has 3 finalists this year! o Johnny Ho o Steven Hao o Jimmy Zeng o That's probably more than any other school this year Officer elections o Check your for the application/instructions later today Stanford ProCo (5/19) registration o KR9EpyErNRlwnbfrq4jmYiM/viewform KR9EpyErNRlwnbfrq4jmYiM/viewform

Stacks and Queues Two Operations: Push and Pop o Push: add an element to your list o Pop: remove an element from your list Stack: Last in, first out o Example o Push 1, Push 5, Push 3, Pop [3] o Push 4, Pop [4] o Pop [5] Queue: First in, first out o Example o Push 1, Push 5, Push 3, Pop[1] o Push 4, Pop[5] o Pop[3]

Singly Linked Lists A data structure that can implement a stack or a queue Supports insertions and deletions at any known location in constant time o Supports pushing to the front, pushing to the back, popping from the front Store a head node and a tail node o Each node has a pointer to the next node o Ex. Push to the front: Make a new node, put it before the head, make it the new head.

Doubly Linked List Like a linked list but supports popping from the back too Stores almost the same thing as a linked list o Each node stores a "forward" pointer to the next node and a "reverse" pointer to the previous node o Ex. Pop from the back: Delete the tail, use the previous node as the new tail

Deque A doubly linked list can implement a deque very easily Deque can push/pop from front or back Useful for job scheduling applications o Threads operate on jobs starting from the front-end of queues o Threads can move jobs from one back-end of a queue to another

Better deque Implementation Circular buffer An array that wraps around the back to the front Store a head and tail in addition to the array This supports: o Constant time pop/push from front/back o Constant time random access

POTW Implement a queue with two stacks. Your program should support the queue operations push(x) and pop() using only two stacks. Sample input: push 5 push 6 push 2 pop push 4 pop push 1 pop Sample output: Constraints: N is the number of instructions (push and pop). 1<=N<=1000: 10 pts 1<=N<=500,000: 15 pts