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Leetcode232-implementQueueUsingStacks

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.

Example

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MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false

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.
  • Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
  • You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Solution

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class MyQueue {
// O(1) amortized
Stack<Integer> input;
Stack<Integer> output;
/** Initialize your data structure here. */
public MyQueue() {
input = new Stack<>();
output = new Stack<>();
}

/** Push element x to the back of queue. */
public void push(int x) {
input.push(x);
}

/** Removes the element from in front of queue and returns that element. */
public int pop() {
peek();
return output.pop();
}

/** Get the front element. */
public int peek() {
if (output.isEmpty())
while(!input.isEmpty())
output.push(input.pop());
return output.peek();
}

/** Returns whether the queue is empty. */
public boolean empty() {
return input.isEmpty() && output.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();
*/