Implement Stack using Queues Solutions in C++

// Source : https://leetcode.com/problems/implement-stack-using-queues/
// Author : Hao Chen
// Date   : 2015-06-13




class Stack {
    public:
        // Push element x onto stack.
        void push(int x) {
            nums.push(x);
        }
        // Removes the element on top of the stack.
        void pop() {
            if (!empty()) {
                int len = nums.size()-1;
                //Don't treat the "-->" is a new operator. ;-)
                while ( len-->0) {
                    nums.push(nums.front());
                    nums.pop();
                }
                nums.pop();
            }
        }
        // Get the top element.
        int top() {
            return nums.back();
        }
        // Return whether the stack is empty.
        bool empty() {
            return nums.empty();
        }

    private:
        queue<int> nums;
};

/// Source : https://leetcode.com/problems/implement-stack-using-queues/description/
/// Author : liuyubobobo
/// Time   : 2018-09-29

#include <iostream>
#include <queue>

using namespace std;


/// Naive Implementation
/// Time Complexity: init: O(1)
///                  push: O(1)
///                  pop: O(n)
///                  top: O(n)
///                  empty: O(1)
/// Space Complexity: O(n)
class MyStack {

private:
    queue<int> q;

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

    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {

        assert(!empty());

        queue<int> q2;
        while(q.size() > 1){
            q2.push(q.front());
            q.pop();
        }

        int ret = q.front();
        q.pop();

        while(!q2.empty()){
            q.push(q2.front());
            q2.pop();
        }

        return ret;
    }

    /** Get the top element. */
    int top() {

        assert(!empty());

        queue<int> q2;
        while(q.size() > 1){
            q2.push(q.front());
            q.pop();
        }

        int ret = q.front();
        q2.push(ret);
        q.pop();

        while(!q2.empty()){
            q.push(q2.front());
            q2.pop();
        }

        return ret;
    }

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


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/implement-stack-using-queues/description/
/// Author : liuyubobobo
/// Time   : 2018-09-29

#include <iostream>
#include <queue>
#include <cassert>

using namespace std;


/// Naive Implementation
/// Using a variable to record top element:)
///
/// Time Complexity: init: O(1)
///                  push: O(1)
///                  pop: O(n)
///                  top: O(1)
///                  empty: O(1)
/// Space Complexity: O(n)
class MyStack {

private:
    queue<int> q;
    int topV;

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

    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        topV = x;
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        assert(!empty());

        queue<int> q2;
        while(q.size() > 1){
            q2.push(q.front());
            q.pop();
        }

        int ret = q.front();
        q.pop();

        while(!q2.empty()){
            q.push(q2.front());
            topV = q2.front();
            q2.pop();
        }

        return ret;
    }

    /** Get the top element. */
    int top() {
        assert(!empty());
        return topV;
    }

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


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/implement-stack-using-queues/description/
/// Author : liuyubobobo
/// Time   : 2018-09-29

#include <iostream>
#include <queue>
#include <cassert>

using namespace std;


/// Naive Implementation
/// Using a variable to record top element:)
///
/// Time Complexity: init: O(1)
///                  push: O(n)
///                  pop: O(1)
///                  top: O(1)
///                  empty: O(1)
/// Space Complexity: O(n)
class MyStack {

private:
    queue<int> q;

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

    /** Push element x onto stack. */
    void push(int x) {

        queue<int> q2;
        while(!q.empty()){
            q2.push(q.front());
            q.pop();
        }

        q.push(x);

        while(!q2.empty()){
            q.push(q2.front());
            q2.pop();
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {

        assert(!empty());
        int ret = q.front();
        q.pop();
        return ret;
    }

    /** Get the top element. */
    int top() {
        assert(!empty());
        return q.front();
    }

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


int main() {

    return 0;
}

/// Source : https://leetcode.com/problems/implement-stack-using-queues/description/
/// Author : liuyubobobo
/// Time   : 2018-09-29

#include <iostream>
#include <queue>
#include <cassert>

using namespace std;


/// Using only one queue!
///
/// Time Complexity: init: O(1)
///                  push: O(n)
///                  pop: O(1)
///                  top: O(1)
///                  empty: O(1)
/// Space Complexity: O(n)
class MyStack {

private:
    queue<int> q;
    int sz = 0;

public:
    /** Initialize your data structure here. */
    MyStack(): sz(0) {}

    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for(int i = 0; i < sz; i ++){
            int e = q.front();
            q.pop();
            q.push(e);
        }
        sz ++;
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {

        assert(!empty());
        int ret = q.front();
        q.pop();
        sz --;
        return ret;
    }

    /** Get the top element. */
    int top() {
        assert(!empty());
        return q.front();
    }

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


int main() {

    return 0;
}