Leetcode 225. Implement stack using Queues using the queue build stack----------java

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 the only standard operations of a queue – which means only push to back ,, peek/pop from front size , and is empty operations AR E valid.
• Depending on your language, queue May is not supported natively. You could simulate a queue by using a list or deque (double-ended queue), as long as if you have standard operations of a Q Ueue.
• You may assume this all operations is valid (for example, no pop or top operations would be called on an empty stack).

The stack consists of a queue, two approaches: 1, one queue implementation, Push is O (n), the other is O (1)

2, two queue implementations, pop is O (n), other things O (1)

classMystack {//One Queue solutionPrivatequeue<integer> q =NewLinkedlist<integer>();//Push element x onto stack. Public voidPushintx) {q.add (x);  for(inti = 1; I < q.size (); i + +) {//rotate the queue to make the tail is the headQ.add (Q.poll ()); }}//removes the element on top of the stack. Public voidpop () {q.poll ();}//Get the top element. Public intTop () {returnQ.peek (); }//Return Whether the stack is empty. Public Booleanempty () {returnq.isempty ();}}

2.

 Public classMystack {PrivateQueue<integer>queue1; PrivateQueue<integer>queue2; Private intnum; /**Initialize your data structure here.*/     PublicMystack () {queue1=NewLinkedList (); Queue2=NewLinkedList (); Num= 1; }        /**Push element x onto stack.*/     Public voidPushintx) {if(num = = 1) {queue1.add (x); } Else{queue2.add (x); }    }        /**removes the element on top of the stack and returns that element.*/     Public intpop () {if(num = = 1){            intSize =queue1.size ();  for(inti = 0; i < size-1; i++) {Queue2.add (Queue1.poll ()); } num= 2; returnQueue1.poll (); } Else {            intSize =queue2.size ();  for(inti = 0; i < size-1; i++) {Queue1.add (Queue2.poll ()); } num= 1; returnQueue2.poll (); }    }        /**Get the top element.*/     Public intTop () {if(num = = 1){            intSize =queue1.size ();  for(inti = 0; i < size-1; i++) {Queue2.add (Queue1.poll ()); }            inttop =Queue1.poll ();            Queue2.add (top); Num= 2; returntop; } Else {            intSize =queue2.size ();  for(inti = 0; i < size-1; i++) {Queue1.add (Queue2.poll ()); }            inttop =Queue2.poll ();            Queue1.add (top); Num= 1; returntop; }      }        /**Returns Whether the stack is empty.*/     Public Booleanempty () {if(num = = 1){            returnQueue1.isempty (); } Else {            returnQueue2.isempty (); }    }}/*** Your Mystack object would be instantiated and called as such: * mystack obj = new Mystack (); * Obj.push (x); * int P Aram_2 = Obj.pop (); * int param_3 = Obj.top (); * Boolean param_4 = Obj.empty (); */

Leetcode 225. Implement stack using Queues using the queue build stack----------java