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implement-stack-using-queues

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Implement Stack using Queues – LeetCode 225

Problem

Description

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.

Answer

Original

Code

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class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for(unsigned i = 0; i != q.size() - 1; ++i){
            int tmp = q.front();
            q.pop();
            q.push(tmp);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int tmp = q.front();
        q.pop();
        return tmp;
    }
    
    /** Get the top element. */
    int top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
private:
    queue<int> q;
};

/**
 * 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();
 */

思路

送分题。
耗时$3$ ms，排名$39.86\%$

Better

思路

针对push操作优化，使用两个queue。
耗时$3$ ms，排名$39.86\%$

Code

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class MyStack {
    int curr_size;
    queue<int> q1,q2;
public:
    /** Initialize your data structure here. */
    MyStack() {
        curr_size =0;
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q1.push(x);
        curr_size++;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if(q1.empty())
            return -1;
        while(q1.size()!=1){
            q2.push(q1.front());
            q1.pop();
        }
        int temp = q1.front();
        q1.pop();
        curr_size--;
        
        queue<int> q = q1;
        q1 = q2;
        q2 = q;
        return temp;
    }
    
    /** Get the top element. */
    int top() {
        return q1.back();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.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();
 */