LeetCode 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).

class Stack {
private:
queue<int> que[2];
int current;

void transfer(queue<int>& que1, queue<int>& que2, int n) {
for (int i=0; i<n; i++) {
que2.push(que1.front());
que1.pop();
}
}
public:
Stack() : current(0){}
// Push element x onto stack.
void push(int x) {
que[current].push(x);
}

// Removes the element on top of the stack.
void pop() {
int nums = que[current].size();
if (nums == 0) {
return;
}
transfer(que[current], que[1 - current], nums - 1);
que[current].pop();
current = 1 - current;
}

// Get the top element.
int top() {
if (empty()) {
return 0;
}
return que[current].back();
}

// Return whether the stack is empty.
bool empty() {
return que[current].empty();
}
};

也可以使用一个队列来完成，不过对于队列来说一个和两个没什么区别只是个头指针而已。