Implement Stack using Queues

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225.Implement Stack using Queues

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:

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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

两个队列：

时间复杂度：O(n)

空间复杂度：O(n)

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class MyStack {
public:
    queue<int> que1;
    queue<int> que2;
    MyStack() {

    }

    void push(int x) {
        que2.push(x);
        while(!que1.empty()) {
            que2.push(que1.front());
            que1.pop();
        }
        swap(que1,que2);
    }

    int pop() {
        int r = que1.front();
        que1.pop();
        return r;
    }

    int top() {
        return que1.front();
    }

    bool empty() {
        return que1.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();
 */

一个队列：

时间复杂度：O(n)

空间复杂度：O(n)

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class MyStack {
public:
    queue<int> que;
    MyStack() {

    }

    void push(int x) {
        int que_size = que.size();
        que.push(x);
        while(que_size--) {
            que.push(que.front());
            que.pop();
        }
    }

    int pop() {
        int r = que.front();
        que.pop();
        return r;
    }

    int top() {
        return que.front();
    }

    bool empty() {
        return que.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();
 */