题目

implement-stack-using-queues

算法

* 直接模拟

* 优化

代码

* 直接模拟

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        q2.push(x);
        while (q2.size() > 1) {
            q1.push(q2.front());
            q2.pop();
        }
    }

    // Removes the element on top of the stack.
    void pop(void) {
        top();
        q2.pop();
    }

    // Get the top element.
    int top(void) {
        if (q2.empty()) {
            for (int i = 0; i < (int)q1.size() - 1; ++i) {
                q1.push(q1.front());
                q1.pop();
            }
            q2.push(q1.front());
            q1.pop();
        }
        return q2.front();
    }

    // Return whether the stack is empty.
    bool empty(void) {
        return q1.empty() && q2.empty();
    }

private:
    queue<int> q1, q2;
};

* 优化

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        queue<int> tmp;
        while (!q.empty()) {
            tmp.push(q.front());
            q.pop();
        }
        q.push(x);
        while (!tmp.empty()) {
            q.push(tmp.front());
            tmp.pop();
        }
    }

    // Removes the element on top of the stack.
    void pop() {
        q.pop();
    }

    // Get the top element.
    int top() {
        return q.front();
    }

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

private:
    queue<int> q;
};