225.implement-stack-using-queues.cpp

/*
 * @lc app=leetcode id=225 lang=cpp
 *
 * [225] Implement Stack using Queues
 *
 * https://leetcode.com/problems/implement-stack-using-queues/description/
 *
 * algorithms
 * Easy (55.77%)
 * Likes:    3028
 * Dislikes: 881
 * Total Accepted:    354.1K
 * Total Submissions: 632.8K
 * 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 stack (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 that 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 using only one queue?
 * 
 */

// @lc code=start
class MyStack {
private:
    #include <queue>
    queue<int> in,out;
public:
    MyStack() {
        
    }
    
    void push(int x) {
        in.push(x);
        for(int i = 0;i<in.size()-1;i++){
            in.push(in.front());
            in.pop();
        }
    }
    
    int pop() {
        int hold = in.front();
        in.pop();
        return hold;
    }
    
    int top() {
        return in.front();
    }
    
    bool empty() {
        return in.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();
 */
// @lc code=end