leetcode 232: Implement Queue using Stacks

leetcode 232: Implement Queue using Stacks

问题描述：

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).

思路：

就是用一个 temp 的 stack 来辅助操作。有两种做法，一种是在长期存储的 stack 存放的是队列顺序，那么 push 操作就需要辅助，pop 和 peek 都不需要；第二种是反过来，长期存储的 stack 存放的仍是栈顺序，那么 push 操作就不需要辅助，pop 和 peek 都需要。

代码：

#include<iostream>
#include<vector>
#include<map>
#include <string>
#include<stack>
using namespace std;
class Queue {
public:

stack<int> queue_by_stack;
stack<int> temp;
// Push element x to the back of queue.
void push(int x) {
while (!queue_by_stack.empty())  //reverse in the temp stack
{
temp.push(queue_by_stack.top());
queue_by_stack.pop();
}

queue_by_stack.push(x);

while (!temp.empty())  //reverse back to the queue stack
{
queue_by_stack.push(temp.top());
temp.pop();
}
}

// Removes the element from in front of queue.
void pop(void) {
if (!queue_by_stack.empty())
queue_by_stack.pop();
}

// Get the front element.
int peek(void) {
if (!queue_by_stack.empty())
{
return queue_by_stack.top();
}
else
return -999;
}

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

int main() //for test
{

Queue test;
test.push(1);
test.push(2);
cout << "Should be 1: " << test.peek() << endl;
test.pop();
if (test.empty())
{
cout << "empty now" << endl;
}

cout << endl << "Should be 2: " << test.peek() << endl;
test.pop();
if (test.empty())
{
cout << "empty now" << endl;
}

return 0;
}