Implement Queue using Stacks

232.Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

• You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

两个堆栈：

时间复杂度：O(1)

空间复杂度：O(n)

class MyQueue {
public:
    stack<int> stk1;
    stack<int> stk2;
    MyQueue() {

    }

    void push(int x) {
        while(!stk1.empty()) {
            stk2.push(stk1.top());
            stk1.pop();
        }
        stk2.push(x);
        while(!stk2.empty()) {
            stk1.push(stk2.top());
            stk2.pop();
        }
    }

    int pop() {
        int value = stk1.top();
        stk1.pop();
        return value;
    }

    int peek() {
        return stk1.top();
    }

    bool empty() {
        return stk1.empty();
    }
};