1.13 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. 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). Show Company Tags Show Tags Show Similar Problems

Method

use the stack's feature first in last out to implement queue so we need two stack to reverse the order

Time and Space Complexity

push o(1) pop o(2n) peek o(2n) isEmpty o(1)

Code

Stack<Integer> s1 = new Stack<Integer>();
Stack<Integer> s2 = new Stack<Integer>();
// Push element x to the back of queue.
public void push(int x) {
    s1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
    if (s1.size() < 2){
        s1.pop();
        return;
    }
    while (!s1.isEmpty()){
          s2.push(s1.pop());
    }
    s2.pop();
    while (!s2.isEmpty()){
         s1.push(s2.pop());
    }
    return;
}

// Get the front element.
public int peek() {
    int res = 0;
    if (s1.size() < 2){
        return s1.peek();
    }
    while (!s1.isEmpty()){
          s2.push(s1.pop());
    }
    res = s2.peek();
    while (!s2.isEmpty()){
         s1.push(s2.pop());
    }
    return res;
}

// Return whether the queue is empty.
public boolean empty() {
     return s1.isEmpty();
}