Title
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).
用队列实现栈，用两个队列实现栈
Solution
//cpp:
class MyStack {
public:
/** Initialize your data structure here. */
queue<int> a, b;
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
a.push(x);
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
while (a.size() > 1) {
b.push(a.front());
a.pop();
}
int num=a.front();
a.pop();
while (!b.empty()) {
a.push(b.front());
b.pop();
}
return num;
}
/** Get the top element. */
int top() {
while (a.size() > 1) {
b.push(a.front());
a.pop();
}
int num = a.front();
b.push(a.front());
a.pop();
while (!b.empty()) {
a.push(b.front());
b.pop();
}
return num;
}
/** Returns whether the stack is empty. */
bool empty() {
return a.size() == 0;
}
};