leetcode :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).

用两个栈实现一个队列的功能

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) {if (!s2.empty())s2.pop();else{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{while (!s1.empty()){s2.push(s1.top());s1.pop();}return s2.top();}}// Return whether the queue is empty.bool empty(void) {return s1.empty() && s2.empty();}};