Title: Implement Stack using Queues Source: leetcode.com
Implement the following operations of a stack using queues.
- push(x) — Push element x onto stack.
- pop() — Removes the element on top of the stack.
- top() — Get the top element.
- empty() — Return whether the stack is empty.
Notes:
- You must use only standard operations of a queue — which means only
push to back,peek/pop from front,size, andis emptyoperations are valid. - Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
- You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
Java solution
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 |
/* https://leetcode.com/problems/implement-stack-using-queues/ */ import java.util.LinkedList; public class ImplementStackUsingQueues { LinkedList<Integer> primary = new LinkedList<Integer>(); int top = -1; // Push element x onto stack. public void push(int x) { primary.add(x); top = x; } // Removes the element on top of the stack. public void pop() { LinkedList<Integer> temp = new LinkedList<Integer>(); int num = -1; while(!primary.isEmpty()) { top = num; num = primary.remove(); if(top != -1) { temp.add(top); } } primary = temp; if(empty()) { top = -1; } temp = null; } // Get the top element. public int top() { return top; } // Return whether the stack is empty. public boolean empty() { if(primary.isEmpty()) { return true; } return false; } public static void main(String args[]) { ImplementStackUsingQueues i = new ImplementStackUsingQueues(); i.push(2); i.push(3); System.out.println(i.primary); i.pop(); System.out.println(i.primary); System.out.println(i.top); i.push(5); i.push(6); System.out.println(i.primary); System.out.println(i.top); i.pop(); System.out.println(i.primary); i.pop(); System.out.println(i.primary); i.pop(); System.out.println(i.primary); i.pop(); } } |