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  • Leetcode题目: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).

    题目解答:

    题目中的要求是使用栈来实现一个队列的功能,具体思路很简单,使用到了两个栈来模拟一个队列的功能。

    代码如下:

    class Queue {
    public:
        // Push element x to the back of queue.
        void push(int x) {
            s1.push(x);
            if(s2.empty())
            {
                while(!s1.empty())
                {
                    s2.push(s1.top());
                    s1.pop();
                }
            }
        }

        // Removes the element from in front of queue.
        void pop(void) {
            if(!s2.empty())
            {
                s2.pop();
            }
            else if(!s1.empty())
            {
                while(!s1.empty())
                {
                    s2.push(s1.top());
                    s1.pop();
                }
                s2.pop();
            }
        }

        // Get the front element.
        int peek(void) {
            if(!s2.empty())
            {
                return s2.top();
            }
            else if(!s1.empty())
            {
                while(!s1.empty())
                {
                    s2.push(s1.top());
                    s1.pop();
                }
                return s2.top();
            }
            return -1;
        }

        // Return whether the queue is empty.
        bool empty(void) {
            if( (s1.empty()) && (s2.empty()) )
                return true;
            else
                return false;
        }
    private:
        stack<int> s1;
        stack<int> s2;
    };

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  • 原文地址:https://www.cnblogs.com/CodingGirl121/p/5425897.html
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