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# Implement Stack by Two Queues

ID: 494; easy

## Solution 1 (Java)

public class Stack {
private Queue<Integer> q1 = new ArrayDeque<>();
private Queue<Integer> q2 = new ArrayDeque<>();
/*
* @param x: An integer
* @return: nothing
*/
public void push(int x) {
q1.offer(x);
}
/*
* @return: nothing
*/
public void pop() {
if (q2.isEmpty())
move();
q1.poll();
swap();
}
/*
* @return: An integer
*/
public int top() {
if (q2.isEmpty())
move();
int topVal = q1.poll();
q2.offer(topVal);
swap();
return topVal;
}
/*
* @return: True if the stack is empty
*/
public boolean isEmpty() {
return q1.isEmpty() && q2.isEmpty();
}
private void move() {
while (q1.size() > 1) {
q2.offer(q1.poll());
}
}
private void swap() {
Queue<Integer> temp = q1;
q1 = q2;
q2 = temp;
}
}
// push(1), push(2), push(3), pop, push(4), top()
// q1 = []
// q2 = [1,2,4]

• LintCode

## Solution 2 (Java)

class MyStack {
Queue<Integer> q1;
Queue<Integer> q2;
/** Initialize your data structure here. */
public MyStack() {
q1 = new ArrayDeque<>();
q2 = new ArrayDeque<>();
}
/** Push element x onto stack. */
public void push(int x) {
q1.offer(x);
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
while (q1.size() > 1) {
q2.offer(q1.poll());
}
int popVal = q1.poll();
moveBack();
return popVal;
}
/** Get the top element. */
public int top() {
while (q1.size() > 1) {
q2.offer(q1.poll());
}
int topVal = q1.peek();
q2.offer(q1.poll());
moveBack();
return topVal;
}
/** Returns whether the stack is empty. */
public boolean empty() {
return q1.isEmpty();
}
private void moveBack() {
while (!q2.isEmpty()) {
q1.offer(q2.poll());
}
}
}
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/

### Notes

• LeetCode
Last modified 2yr ago