225.ImplementStackUsingQueues.cpp

/*
 * @lc app=leetcode id=225 lang=cpp
 *
 * [225] Implement Stack using Queues
 *
 * https://leetcode.com/problems/implement-stack-using-queues/description/
 *
 * algorithms
 * Easy (47.30%)
 * Likes:    1032
 * Dislikes: 680
 * Total Accepted:    218K
 * Total Submissions: 454.3K
 * Testcase Example:
 * '["MyStack","push","push","top","pop","empty"]\n[[],[1],[2],[],[],[]]'
 *
 * Implement a last in first out (LIFO) stack using only two queues. The
 * implemented stack should support all the functions of a normal queue (push,
 * top, pop, and empty).
 *
 * Implement the MyStack class:
 *
 *
 * void push(int x) Pushes element x to the top of the stack.
 * int pop() Removes the element on the top of the stack and returns it.
 * int top() Returns the element on the top of the stack.
 * boolean empty() Returns true if the stack is empty, false otherwise.
 *
 *
 * Notes:
 *
 *
 * You must use only standard operations of a queue, which means only push to
 * back, peek/pop from front, size, and is empty operations are valid.
 * Depending on your language, the queue may not be supported natively. You may
 * simulate a queue using a list or deque (double-ended queue), as long as you
 * use only a queue's standard operations.
 *
 *
 *
 * Example 1:
 *
 *
 * Input
 * ["MyStack", "push", "push", "top", "pop", "empty"]
 * [[], [1], [2], [], [], []]
 * Output
 * [null, null, null, 2, 2, false]
 *
 * Explanation
 * MyStack myStack = new MyStack();
 * myStack.push(1);
 * myStack.push(2);
 * myStack.top(); // return 2
 * myStack.pop(); // return 2
 * myStack.empty(); // return False
 *
 *
 *
 * Constraints:
 *
 *
 * 1 <= x <= 9
 * At most 100 calls will be made to push, pop, top, and empty.
 * All the calls to pop and top are valid.
 *
 *
 *
 * Follow-up: Can you implement the stack such that each operation is amortized
 * O(1) time complexity? In other words, performing n operations will take
 * overall O(n) time even if one of those operations may take longer. You can
 * use more than two queues.
 *
 */

// @lc code=start
#include <queue>

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

    /** Push element x onto stack. */
    void push(int x) {
        std::queue<int> temp;
        temp.push(x);
        while (!ordered_.empty()) {
            temp.push(ordered_.front());
            ordered_.pop();
        }
        ordered_.swap(temp);
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val = ordered_.front();
        ordered_.pop();
        return val;
    }

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

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

private:
    std::queue<int> ordered_;
};

/**
 * 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();
 */
// @lc code=end