队列的实现

• 232. Implement Queue using Stacks
  • 思路
  • C++解法
  • Java解法

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.

Follow-up: Can you implement the queue 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.

思路

用两个栈实现队列。当stackOut为空时就把stackIn中所有元素加入到stackOut中。

因为All the calls to pop and peek are valid.所以这里不再需要校验操作的合法性。

C++解法

class MyQueue {
public:
    stack<int> stackIn;
    stack<int> stackOut;
    MyQueue() {}

    void push(int x) { stackIn.push(x); }

    int pop() {
        if (stackOut.empty()) {
            while (!stackIn.empty()) {
                int x = stackIn.top();
                stackIn.pop();
                stackOut.push(x);
            }
        }
        int result = stackOut.top();
        stackOut.pop();
        return result;
    }

    int peek() {
        int result = this->pop();
        stackOut.push(result);
        return result;
    }

    bool empty() { return stackOut.empty() && stackIn.empty(); }
};

/**
 * 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();
 */

Java解法