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Implement Stack using Queues⚓︎

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Description⚓︎

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.

Example 1:

  • Input
    • ["MyStack", "push", "push", "top", "pop", "empty"]
    • [[], [1], [2], [], [], []]
  • Output
    • [null, null, null, 2, 2, false]

Explanation

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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 to push, pop, top, and empty.
  • All the calls to pop and top are valid.

Solution⚓︎

Implementation Using Queues⚓︎

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class MyStack {
public:
    queue<int> q, sq;
    MyStack() {

    }

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

    int pop() {
        while (q.size() > 1) {
            sq.push(q.front());
            q.pop();
        }
        int res = q.front();
        q.pop();
        while (!sq.empty()) {
            q.push(sq.front());
            sq.pop();
        }
        return res;
    }

    int top() {
        return q.back();
    }

    bool empty() {
        return q.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();
 */

Implementation Using Array⚓︎

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class MyStack {
public:
    MyStack() {
        tt = 0;
    }

    void push(int x) {
        stk[++tt] = x;
    }

    int pop() {
        return stk[tt--];
    }

    int top() {
        return stk[tt];
    }

    bool empty() {
        return tt <= 0;
    }
    int stk[110];
    int tt;
};

/**
 * 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();
 */