Implement Stack using Queues

Description

Implement Stack using Queues - LeetCode

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() Returns true 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 and is 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.

Examples

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

• $1<=\text{x}<=9$
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

Code

# include <queue>
# include <vector>
using namespace std;
 
class MyStack {
public:
    queue<int> q;
    
    MyStack() {}
 
    void push(int x) {
        if (q.empty()){
            q.push(x);
        } else {
            vector<int> array;
            while (!q.empty()){ array.push_back(q.front()); q.pop(); }
            q.push(x);
            for (int i = 0; i < array.size(); i++) { q.push(array[i]); }
        }
    }
 
    int pop() {
        int result = q.front();
        q.pop();
        return result;
    }
 
    int top() {
        return q.front();
    }
 
    bool empty() {
        return q.empty();
    }
};

Approach

1. Create a queue as per the description
2. push member function
   • Check if the queue is empty
     • Push to Queue
   • Otherwise
     • Create a local array for temporary storage
     • Populate the array based on the front of the queue and maintain the order
     • Push the front(top) into the queue
     • Restore the values from the array into the queue
3. pop member function
   • Create a temporary variable storing the top of the queue
   • Pop the front(top) of the queue
   • Return the temporary variable
4. top member function
   • Return the front(top) of the queue
5. empty member function
   • Return whether the queue is empty by calling the queue STL function empty()