[leetcode] 132.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).

题意：

使用栈来实现队列。

思路：

使用两个栈，一个用来push数据，另一个的作用是查看队列的最先进入的数据以及pop出第一个数据。

两个栈记为s1,s2,入队列时存入s1中，出队列时或者查看第一个元素时，查看s2中是否有元素，没有的话从s1中将元素放入到s2中，由于元素进入s1是1,2..n，而进入s2时是n…2,1所以接下来出s2的顺序就是进入s1的顺序。

以上，代码如下：

class Queue {
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()) {
if(s1.empty())return;
while(!s1.empty()) {
s2.push(s1.top());
s1.pop();
}
}
s2.pop();
}

// Get the front element.
int peek(void) {
if(s2.empty()) {
if(s1.empty())return -1;
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()));
}
private:
stack<int> s1;
stack<int> s2;
};