232. Implement Queue using Stacks
Two Stacks.
Stack 1 is the only stack to store new elements when adding a new element into the queue.
Stack 2 is the only stack to pop old element out of the queue. When stack2 is empty, we move all data from Stack1 to Stack2.
Push - O(1) per operation, Pop - Amortized O(1) per operation.
class MyQueue {
private Deque<Integer> stack1;
private Deque<Integer> stack2;
/** Initialize your data structure here. */
public MyQueue() {
stack1 = new ArrayDeque<>();
stack2 = new ArrayDeque<>();
}
/** Push element x to the back of queue. */
public void push(int x) {
stack1.offerFirst(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if (stack2.isEmpty()) {
while (!stack1.isEmpty()) {
stack2.offerFirst(stack1.pollFirst());
}
}
return stack2.pollFirst();
}
/** Get the front element. */
public int peek() {
if (stack2.isEmpty()) {
while (!stack1.isEmpty()) {
stack2.offerFirst(stack1.pollFirst());
}
}
return stack2.peekFirst();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack1.isEmpty() && stack2.isEmpty();
}
}
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* boolean param_4 = obj.empty();
*/