Leetcode-Implement Stack using Queues（C++）

Implement the following operations of a stack using queues.

push(x) – Push element x onto stack.

pop() – Removes the element on top of the stack.

top() – Get the top element.

empty() – Return whether the stack is empty.

Notes:

You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.

Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.

You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题意：利用queue来设计一个stack，实现push，top，pop与empty

解题思路：关键在于保持新插入的数永远在queue的最前面。这样top以及pop的时候可以轻松操作。

建立两个queue，始终保持一个为空。

push：将插入值push入空的queue1，然后将另一个非空queue2中的数值依次弹出放入queue1.

class Stack {
public:
queue<int> q[2];
int t=1;
// Push element x onto stack.
void push(int x) {
q[1-t].push(x);
while(!q[t].empty())
{
q[1-t].push(q[t].front());
q[t].pop();
}
t=1-t;//利用1-0=1和1-1=0来循环操作queue
}

// Removes the element on top of the stack.
void pop() {
if(!q[t].empty())
{
q[t].pop();
}

}

// Get the top element.
int top() {
if(!q[t].empty())
{
return q[t].front();
}

}

// Return whether the stack is empty.
bool empty() {
return q[t].empty();

}
};