leetcode代码记录 232 Implement Queue using Stacks

最新推荐文章于 2021-03-11 21:01:10 发布

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本文介绍了一种使用两个栈来模拟队列行为的方法。通过一个栈进行元素的压入操作，另一个栈则用于弹出和获取队首元素，确保了先进先出的原则。这种方法巧妙地利用了栈的特性，实现了队列的基本操作。

题目如下：

232. Implement Queue using Stacks

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).

解题思想，使用两个栈代替队列，一个进，一个出。代码如下：

class Queue {
public:

    stack<int> stkin;
    stack<int> stkout;
    // Push element x to the back of queue.
    void push(int x) {
        stkin.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if(stkout.empty()) {
            if(stkin.empty()) {
                return ;
            }
            int size = stkin.size();
            for(int i = 0; i < size; i++) {
                stkout.push(stkin.top());
                stkin.pop();
            }
        }
        stkout.pop();
    }

    // Get the front element.
    int peek(void) {
         if(stkout.empty()) {
            if(stkin.empty()) {
                return 0;
            }
            int size = stkin.size();
            for(int i = 0; i < size; i++) {
                stkout.push(stkin.top());
                stkin.pop();
            }
        }
        return (stkout.top());
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return(stkin.empty() && stkout.empty());
    }
};