LeetCode232—Implement Queue using Stacks-蒲公英云

LeetCode232—Implement Queue using Stacks

原题

原题链接

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).

分析

栈是后进先出，队列是先进先出。

如果用栈实现队列的话，需要用到两个栈

假设两个栈stack1和stack2，假设现在一组数据压栈，那么他们出栈的顺序肯定是相反的，也就是所谓的后进先出。现在我们将数据压入stack1然后出栈的时候再把数据压入stack2，那么原先stack1中的栈底元素就成了stack2的栈顶元素，弹出的时候也就“先出了”，这就用两个栈实现了一个队列。

代码

class Queue {
private:
    stack<int>s1;
    stack<int>s2;
public:
    // Push element x to the back of queue.
    void push(int x) {
        s1.push(x);
    }
    // Removes the element from in front of queue.
    void pop(void) {
        while(!s1.empty())
        {
            s2.push(s1.top());
            s1.pop();
        }
        s2.pop();
        while(!s2.empty())
        {
            s1.push(s2.top());
            s2.pop();
        }
    }
    // Get the front element.
    int peek(void) {
        while(!s1.empty())
        {
            s2.push(s1.top());
            s1.pop();
        }
        int t = s2.top();
        while(!s2.empty())
        {
            s1.push(s2.top());
            s2.pop();
        }
        return t;
    }
    // Return whether the queue is empty.
    bool empty(void) {
        return s1.empty();
    }
};