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Question

Implement the following operations of a stack using queues.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• empty() – Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Solution

Result: Accepted Time: 0 ms

Here should be some explanations.

class Stack {
    queue<int> que;
public:
    // Push element x onto stack.
    void push(int x) {
        que.push(x);

    }

    // Removes the element on top of the stack.
    void pop() {
        int n = que.size();
        if(n<1) return ;
        while(--n)
        {
            que.push(que.front());
            que.pop();
        }
        que.pop();
    }

    // Get the top element.
    int top() {
        return que.back();
    }

    // Return whether the stack is empty.
    bool empty() {
        return que.empty();
    }
};

Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(n)$