225. Implement Stack using Queues
Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).
Implement the MyStack class:
- void push(int x) Pushes element x to the top of the stack.
- int pop() Removes the element on the top of the stack and returns it.
- int top() Returns the element on the top of the stack.
- boolean empty() Returns true if the stack is empty, false otherwise.
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, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue's standard operations.
Example 1:
Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]
Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False
Constraints:
- 1 <= x <= 9
- At most 100 calls will be made to push, pop, top, and empty.
- All the calls to pop and top are valid.
Follow-up: Can you implement the stack such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer. You can use more than two queues.
class MyStack {
Queue<Integer> queue1 = new LinkedList<>();
Queue<Integer> queue2 = new LinkedList<>();
/** Initialize your data structure here. */
public MyStack() {
}
/** Push element x onto stack. */
public void push(int x) {
if(!queue1.isEmpty()) {
queue1.offer(x);
}else {
queue2.offer(x);
}
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
if(queue1.isEmpty()) {
while(queue2.size()>1){
queue1.offer(queue2.poll());
}
return queue2.poll();
}else {
while(queue1.size()>1){
queue2.offer(queue1.poll());
}
return queue1.poll();
}
}
/** Get the top element. */
public int top() {
if(queue1.isEmpty()) {
while(queue2.size()>1){
queue1.offer(queue2.poll());
}
int result = queue2.peek();
queue1.offer(queue2.poll());
return result;
}else {
while(queue1.size()>1){
queue2.offer(queue1.poll());
}
int result = queue1.peek();
queue2.offer(queue1.poll());
return result;
}
}
/** Returns whether the stack is empty. */
public boolean empty() {
return queue1.isEmpty()&&queue2.isEmpty();
}
}
/**
* 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();
*/