Problem
Implement a MyQueue class which implements a queue using two stacks.
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).
Example
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // return 1
queue.pop(); // return 1
queue.empty(); // return false
Solution
栈s1用于push，s2用于peek和pop。
peek和pop的返回值都来自s2：
- 当s2中有元素时，直接返回栈顶元素；
- 当s2中没有元素值，就将s1中的元素全部弹出，并压入s2，然后返回栈顶元素
class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
while(!s1.empty())
{
s1.pop();
}
while(!s2.empty())
{
s2.pop();
}
}
/** Push element x to the back of queue. */
void push(int x) {
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
if(!s2.empty())
{
int tmp = s2.top();
s2.pop();
return tmp;
}
else
{
while(!s1.empty())
{
int tmp = s1.top();
s1.pop();
s2.push(tmp);
}
int tmp = s2.top();
s2.pop();
return tmp;
}
}
/** Get the front element. */
int peek() {
if(!s2.empty())
{
return s2.top();
}
else
{
while(!s1.empty())
{
int tmp = s1.top();
s1.pop();
s2.push(tmp);
}
return s2.top();
}
}
/** Returns whether the queue is empty. */
bool empty() {
return s1.empty() && s2.empty();
}
stack<int> s1;
stack<int> s2;
};
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue* obj = new MyQueue();
* obj->push(x);
* int param_2 = obj->pop();
* int param_3 = obj->peek();
* bool param_4 = obj->empty();
*/
