Question
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).
Solution
TODO
Code
class MyQueue {
Stack<Integer> pushStack = new Stack();
Stack<Integer> popStack = new Stack();
/** Initialize your data structure here. */
public MyQueue() {
}
/** Push element x to the back of queue. */
public void push(int x) {
pushStack.push(x);
}
public void preparePopStack() {
while(!pushStack.empty()) {
popStack.push(pushStack.pop());
}
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if(empty()) return -1;
if(popStack.empty()) {
preparePopStack();
}
return popStack.pop();
}
/** Get the front element. */
public int peek() {
if(empty()) return -1;
if(popStack.empty()) {
preparePopStack();
}
return popStack.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return pushStack.empty() && popStack.empty();
}
}
Performance
TODO