leetcode-225-Implement Stack using Queues

Implement Stack using Queues

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).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.
用队列实现栈。

用两个队列即可实现栈。

class Stack {
public:
queue<int>q1,q2;
// Push element x onto stack.
void push(int x) {
q1.push(x);
}

// Removes the element on top of the stack.
void pop() {
while (!q1.empty()) {
if (q1.size() > 1)
q2.push(q1.front());
q1.pop();
}
while (!q2.empty()) {
q1.push(q2.front());
q2.pop();
}
}

// Get the top element.
int top() {
return q1.back();
}

// Return whether the stack is empty.
bool empty() {
return q1.empty();
}
};