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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

Constraints:

  • 1 <= x <= 9
  • At most 100 calls will be made to push, pop, peek, and empty.
  • All the calls to pop and peek are valid.

解法：使用兩個 stacks

image

class MyQueue {
public:
    MyQueue() {}

    void push(int x) {
        // 後進來的會維持在最上層
        // 如果 s1 推了 1, 2, 3，3 會在最上面
        s1.push(x);
    }

    int pop() {
        peek();
        int temp = s2.top(); s2.pop();
        return temp;
    }

    int peek() {
        // s2 用來儲存 s1 的反轉順序，如果 s2 裡面有值，直接回傳 s2.top();
        if(!s2.empty()) return s2.top();
        // 如果 s1 還有值，將元素丟到 s2 儲存，反轉順序
        while(!s1.empty()) {
            s2.push(s1.top()); s1.pop();
        }
        return s2.top();
    }

    bool empty() {
        return s1.empty() && s2.empty();
    }
private:
    stack<int> s1; // push 進去的元素先裝在這，最後一個元素會維持在最上層
    stack<int> s2; // 將 s1 的元素 pop() 到 s2，反轉順序
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */
  • T: O(N)O(N)
  • S: O(N)O(N)