# LeetCode 232 uses a stack to implement a queue, 225 uses a queue to implement a stack

#### 232. Implement a queue with a stack

##### topic:

Please use only two stacks to implement a first-in-first-out queue. The queue should support all operations supported by general queues (push, pop, peek, empty):

Implement the MyQueue class:

• void push(int x) pushes element x to the end of the queue
• int pop() removes and returns an element from the beginning of the queue
• int peek() returns the element at the head of the queue
• boolean empty() returns true if the queue is empty; otherwise, returns false

illustrate:

• You can only use standard stack operations - that is, only push to top, peek/pop from top, size, and is empty operations are legal.
• Your language may not support stacks. You can use list or deque (double-ended queue) to simulate a stack, as long as it is a standard stack operation.
##### Example 1:
```enter:
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
output:
[null, null, null, 1, 1, false]

explain:
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

```
##### hint:
• 1 <= x <= 9

• Up to 100 calls to push, pop, peek and empty

• Assume all operations are valid (e.g. an empty queue will not invoke pop or peek operations)

• Can you implement a queue with O(1) amortized time complexity per operation? In other words, the total time complexity to perform n operations is O(n) even though one of the operations may take longer.

Stack: The stack is a last-in-first-out data structure, elements are pushed into the stack from the top, and then popped from the top

Queue: A queue is a first-in, first-out data structure. Elements enter the queue from the back end and then dequeue from the front end.

##### c++ code implementation
```class MyQueue {
public:

stack<int> inStack, outStack;

MyQueue() {

}

void push(int x) {
inStack.push(x);
}

int pop() {
if (outStack.empty()){
while (!inStack.empty()) {
outStack.push(inStack.top());
inStack.pop();
}
}
int x = outStack.top();
outStack.pop();
return x;
}

int peek() {
if (outStack.empty()) {
while (!inStack.empty()) {
outStack.push(inStack.top());
inStack.pop();
}
}
return outStack.top();
}

bool empty() {
return inStack.empty() && outStack.empty();
}
};

/**
* 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();
* bool param_4 = obj->empty();
*/
```
##### python code implementation
```class MyStack:
def __init__(self):
self.stack = []

def top(self):
return self.stack[-1]

def push(self, x:int) -> None:
self.stack.append(x)

def pop(self) -> int:
return self.stack.pop()

def empty(self) -> bool:
return (len(self.stack) == 0)

class MyQueue:

def __init__(self):
self.inStack = MyStack()
self.outStack = MyStack()

def push(self, x: int) -> None:
self.inStack.push(x)

def pop(self) -> int:
if self.outStack.empty():
while self.inStack.empty() != True:
self.outStack.push(self.inStack.top())
self.inStack.pop()
x = self.outStack.top()
self.outStack.pop()
return x

def peek(self) -> int:
if self.outStack.empty():
while self.inStack.empty() != True:
self.outStack.push(self.inStack.top())
self.inStack.pop()
return self.outStack.top()

def empty(self) -> bool:
return self.inStack.empty() and  self.outStack.empty()

# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()
```

#### 225 Implementing a stack with a queue

##### topic:

Please use only two queues to implement a last-in-first-out (LIFO) stack, and support all four operations (push, top, pop, and empty) of a normal stack.

Implement the MyStack class:

• void push(int x) pushes the element x onto the top of the stack.
• int pop() removes and returns the top element of the stack.
• int top() Returns the top element of the stack.
• boolean empty() Returns true if the stack is empty; otherwise, returns false .

Notice:

• You can only use the basic operations of the queue - namely push to back, peek/pop from front, size and is empty.
• Your language may not support queues. You can use list (list) or deque (double-ended queue) to simulate a queue, as long as it is a standard queue operation.
##### Example:
```enter:
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
output:
[null, null, null, 2, 2, false]

explain:
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // returns False
```
##### hint:
• 1 <= x <= 9
• Up to 100 calls to push, pop, top and empty
• Each call to pop and top ensures that the stack is not empty
##### c++ code implementation
```class MyStack {
public:
queue<int> inDeque;

MyStack() {
}

void push(int x) {
int n = inDeque.size();
inDeque.push(x);
for (int i = 0; i < n; i++) {
inDeque.push(inDeque.front());
inDeque.pop();
}
}

int pop() {
int x = inDeque.front();
inDeque.pop();
return x;
}

int top() {
return inDeque.front();
}

bool empty() {
return inDeque.empty();
}
};

/**
* Your MyStack object will be instantiated and called as such:
* MyStack* obj = new MyStack();
* obj->push(x);
* int param_2 = obj->pop();
* int param_3 = obj->top();
* bool param_4 = obj->empty();
*/
```
##### python code implementation
```class MyStack:

def __init__(self):
self.queque = collections.deque()

def push(self, x: int) -> None:
n = len(self.queque)
self.queque.append(x)
for i in range(n):
self.queque.append(self.queque.popleft())

def pop(self) -> int:
return self.queque.popleft()

def top(self) -> int:
return self.queque[0]

def empty(self) -> bool:
return not self.queque

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()
```

Posted by planethax on Wed, 16 Nov 2022 02:28:04 +0300