【LeetCode】232. Implement Queue using Stacks

问题描述

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.

You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

用栈实现队列。

代码

class MyQueue {
public:
    stack<int> queue;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        stack<int> tmp;
        int len = queue.size();
        for(int i=0;i<len;i++) {
            int t = queue.top();
            queue.pop();
            tmp.push(t);
        }
        tmp.push(x);
        len = tmp.size();
        for(int i=0;i<len;i++) {
            int t = tmp.top();
            tmp.pop();
            queue.push(t);
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int r = queue.top();
        queue.pop();
        return r;
    }
    
    /** Get the front element. */
    int peek() {
        return queue.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return queue.empty();
    }
};