Implement Stack using Queues Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push
, top
, pop
, and empty
).
Implement the MyStack
class:
void push(int x)
Pushes element x to the top of the stack.int pop()
Removes the element on the top of the stack and returns it.int top()
Returns the element on the top of the stack.boolean empty()
Returnstrue
if the stack is empty,false
otherwise.
Notes:
- You must use only standard operations of a queue, which means that only
push to back
,peek/pop from front
,size
andis empty
operations are valid. - Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue’s standard operations.
Example 1:
Input ["MyStack", "push", "push", "top", "pop", "empty"] [[], [1], [2], [], [], []] Output[null, null, null, 2, 2, false]
Explanation MyStack myStack = new MyStack(); myStack.push(1); myStack.push(2); myStack.top(); // return 2 myStack.pop(); // return 2 myStack.empty(); // return False
Constraints:
1 <= x <= 9
- At most
100
calls will be made topush
,pop
,top
, andempty
. - All the calls to
pop
andtop
are valid.
Implement Stack using Queues Solutions
✅Time: O(1)
✅Space: O(n)
C++
class MyStack {
public:
void push(int x) {
q.push(x);
for (int i = 0; i < q.size() - 1; ++i) {
q.push(q.front());
q.pop();
}
}
int pop() {
const int val = q.front();
q.pop();
return val;
}
int top() {
return q.front();
}
bool empty() {
return q.empty();
}
private:
queue<int> q;
};
Java
class MyStack {
public void push(int x) {
q.offer(x);
for (int i = 0; i < q.size() - 1; ++i)
q.offer(q.poll());
}
public int pop() {
return q.poll();
}
public int top() {
return q.peek();
}
public boolean empty() {
return q.isEmpty();
}
private Queue<Integer> q = new ArrayDeque<>();
}
Python
class MyStack:
def __init__(self):
self.q = deque()
def push(self, x: int) -> None:
self.q.append(x)
for _ in range(len(self.q) - 1):
self.q.append(self.q.popleft())
def pop(self) -> int:
return self.q.popleft()
def top(self) -> int:
return self.q[0]
def empty(self) -> bool:
return not self.q
Watch Tutorial
Checkout more Solutions here