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232. Queue using Stacks
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232. Queue using Stacks
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/*
https://leetcode.com/problems/implement-queue-using-stacks/description/
Implement the following operations of a queue using stacks.
push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.
Notes:
You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
*/
class MyQueue {
private Stack<Integer> stack;
int size = 0;
/**
* Initialize your data structure here.
*/
public MyQueue() {
stack = new Stack<>();
}
/**
* Push element x to the back of queue.
*/
public void push(int x) {
stack.push(x);
size++;
}
/**
* Removes the element from in front of queue and returns that element.
*/
public int pop() {
Stack<Integer> newsStack = new Stack<>();
for (int i = 0; i < size - 1; i++) {
newsStack.push(stack.pop());
}
int first = stack.pop();
size--;
for (int i = 0; i < size; i++) {
stack.push(newsStack.pop());
}
return first;
}
/**
* Get the front element.
*/
public int peek() {
Stack<Integer> newsStack = new Stack<>();
for (int i = 0; i < size - 1; i++) {
newsStack.push(stack.pop());
}
int first = stack.pop();
stack.push(first);
for (int i = 0; i < size - 1; i++) {
stack.push(newsStack.pop());
}
return first;
}
/**
* Returns whether the queue is empty.
*/
public boolean empty() {
return stack.isEmpty();
}
}