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MyStack.java
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94 lines (87 loc) · 3.14 KB
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// Source : https://leetcode.com/problems/implement-stack-using-queues/description/
// Author : Tianming Cao
// Date : 2018-02-02
/**********************************************************************************
* 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, and is empty operations 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).
*
**********************************************************************************/
package myStack;
import java.util.LinkedList;
import java.util.Queue;
/**********************************************************************************
* This problem is a sibling of problem 232(https://leetcode.com/problems/implement-queue-using-stacks/description/).
* The train of thought is:
* If we want to pop a element,first we should judge which queue is not empty assumed queue1,another empty queue assumed queue2.
* Next,we pop all element from queue1 and push them into queue2 util queue1's size is 1.
* Finally the last element in queue1 is the correct pop value.
* If queue1 is empty and queue2 is not empty,the step is symmetric.
* The top operation is similar with pop operation.
**********************************************************************************/
public class MyStack {
public Queue<Integer> queue1;
public Queue<Integer> queue2;
public int flag;
public int size;
public MyStack() {
queue1=new LinkedList<Integer>();
queue2=new LinkedList<Integer>();
flag=1;
size=0;
}
public void push(int x) {
if(flag==1){
queue1.offer(x);
}else{
queue2.offer(x);
}
size++;
}
public int pop() {
int value;
if(flag==1){
while(queue1.size()>1){
queue2.offer(queue1.poll());
}
value=queue1.poll();
flag=2;
}else{
while(queue2.size()>1){
queue1.offer(queue2.poll());
}
value=queue2.poll();
flag=1;
}
size--;
return value;
}
public int top() {
if(flag==1){
while(queue1.size()>1){
queue2.offer(queue1.poll());
}
int value=queue1.poll();
queue2.offer(value);
flag=2;
return value;
}else{
while(queue2.size()>1){
queue1.offer(queue2.poll());
}
int value=queue2.poll();
queue1.offer(value);
flag=1;
return value;
}
}
public boolean empty() {
return size==0?true:false;
}
}