leetcode代码记录 232 Implement Queue using Stacks

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());    }};