LeetCode（232） Implement Queue using Stacks

题目

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

用栈实现队列；

只需要两个辅助栈，一个保存压入元素，一个保存弹出元素；当弹出元素栈空时，把所有入栈元素压入出栈；

AC代码

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        inStack.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if (outStack.empty())
        {
            while (!inStack.empty())
            {
                outStack.push(inStack.top());
                inStack.pop();
            }
        }
        outStack.pop();
    }

    // Get the front element.
    int peek(void) {
        if (outStack.empty())
        {
            while (!inStack.empty())
            {
                outStack.push(inStack.top());
                inStack.pop();
            }
        }
        return outStack.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        if (outStack.empty() && inStack.empty())
            return true;
        return false;
    }

private:
    stack<int> inStack;
    stack<int> outStack;
};

GitHub测试程序源码