Cinte's Leetcode Record
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225. 用队列实现栈

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Symbols count in article: 2.1k Reading time ≈ 2 mins.

225. 用队列实现栈

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class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {

    }
    
    /** Push element x onto stack. */
    // O(1)
    void push(int x) {
        if(!q1.empty())
            q1.push(x);
        else
            q2.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    // O(n)
    int pop() {
        int ret;
        if(!q1.empty())
        {
            while(q1.size() != 1)
            {
                q2.push(q1.front());
                q1.pop();
            }
            ret = q1.front();
            q1.pop();
        }else
        {
            while(q2.size() != 1)
            {
                q1.push(q2.front());
                q2.pop();
            }
            ret = q2.front();
            q2.pop();
        }
        return ret;
    }
    
    /** Get the top element. */
    // O(n)
    int top() {
        int ret;
        if(!q1.empty())
        {
            while(q1.size() != 1)
            {
                q2.push(q1.front());
                q1.pop();
            }
            ret = q1.front();
            q2.push(q1.front());
            q1.pop();
        }else
        {
            while(q2.size() != 1)
            {
                q1.push(q2.front());
                q2.pop();
            }
            ret = q2.front();
            q1.push(q2.front());
            q2.pop();
        }
        return ret;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty() && q2.empty();
    }
private:
    queue<int> q1;
    queue<int> q2;
};

/**
 * 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();
 */

另一种

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class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {

    }
    
    /** Push element x onto stack. */
    // O(n)
    void push(int x) {
        q2.push(x);
        while(!q1.empty())
        {
            q2.push(q1.front());
            q1.pop();
        }
        swap(q1, q2);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    // O(1)
    int pop() {
        auto ret = q1.front();
        q1.pop();
        return ret;
    }
    
    /** Get the top element. */
    // O(1)
    int top() {
        return q1.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty();
    }
private:
    queue<int> q1;
    queue<int> q2;
};

/**
 * 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();
 */