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Implement Queue using Stacks – LeetCode 232

Problem

Description

Implement the following operations of a queue using stacks.

  • push(x) – Push element x to the back of queue.

  • pop() – Removes the element from in front of queue.

  • peek() – Get the front element.

  • empty() – Return whether the queue is empty.

Answer

Original

Code

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class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        while(!s1.empty())
            {s2.push(s1.top()); s1.pop();}
        s1.push(x);
        while(!s2.empty())
            {s1.push(s2.top()); s2.pop();}
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int tmp = s1.top();
        s1.pop();
        return tmp;
    }
    
    /** Get the front element. */
    int peek() {
        return s1.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty();
    }
private:
     stack<int> s1, s2;
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */

思路

在push时维护queue的顺序，时间复杂度$O(n)$，空间复杂度$O(n)$。
耗时$3$ ms，排名$39.44\%$

Better

思路

理论上更好的方案，针对push多的情况，而且pop是严格一倍n，性能理应更强，时间复杂度$O(n)$，空间复杂度$O(n)$。
耗时$3$ ms，排名$39.44\%$

Code

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class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue():front(0) {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        if(s1.empty()) 
            front = x;
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(s2.empty()){
            while(!s1.empty())
                {s2.push(s1.top()); s1.pop();}
        }
        int tmp = s2.top();
        s2.pop();
        return tmp;
    }
    
    /** Get the front element. */
    int peek() {
        if(!s2.empty())
            return s2.top();
        return front;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty();
    }
private:
    stack<int> s1, s2;
    int front;
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */