0232. Implement Queue using Stacks
https://leetcode.com/problems/implement-queue-using-stacks
Description
Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push
, peek
, pop
, and empty
).
Implement the MyQueue
class:
void push(int x)
Pushes element x to the back of the queue.int pop()
Removes the element from the front of the queue and returns it.int peek()
Returns the element at the front of the queue.boolean empty()
Returnstrue
if the queue is empty,false
otherwise.
Notes:
You must use only standard operations of a stack, which means only
push to top
,peek/pop from top
,size
, andis empty
operations are valid.Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.
Example 1:
**Input**
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
**Output**
[null, null, null, 1, 1, false]
**Explanation**
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false
Constraints:
1 <= x <= 9
At most
100
calls will be made topush
,pop
,peek
, andempty
.All the calls to
pop
andpeek
are valid.
Follow-up: Can you implement the queue such that each operation is amortized O(1)
time complexity? In other words, performing n
operations will take overall O(n)
time even if one of those operations may take longer.
ac
class MyQueue {
Stack<Integer> stack, stack2;
/** Initialize your data structure here. */
public MyQueue() {
stack = new Stack<>();
stack2 = new Stack<>();
}
/** Push element x to the back of queue. */
public void push(int x) {
stack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if (stack2.isEmpty()) {
while (!stack.isEmpty())
stack2.push(stack.pop());
}
return stack2.pop();
}
/** Get the front element. */
public int peek() {
if (stack2.isEmpty()) {
while (!stack.isEmpty())
stack2.push(stack.pop());
}
return stack2.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack.isEmpty() && stack2.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();
*/
/*
just 2 stack, when pop()/peek(), pour all elements from stack1 to stack2
*/
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