Problem_232. Implement Queue using Stacks - xwu36/LeetCode GitHub Wiki

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. 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).

code:
class MyQueue {

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