LeetCode: 232. Implement Queue using Stacks
题目描述
Implement the following operations of a queue using stacks.
push(x)
– Push element x to the back of queue.
pop()
– Removes the element from in front of queue.
peek()
– Get the front element.
empty()
– Return whether the queue is empty.
Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false
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).
解题思路
用两个栈来模拟队列。
AC 代码
class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
m_stk.push(x);
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
if(m_que.empty())
{
MoveStk2Que();
}
int front = m_que.top();
m_que.pop();
return front;
}
/** Get the front element. */
int peek() {
if(m_que.empty())
{
MoveStk2Que();
}
return m_que.top();
}
/** Returns whether the queue is empty. */
bool empty() {
return (m_stk.empty() && m_que.empty());
}
private:
void MoveStk2Que()
{
while(!m_stk.empty())
{
m_que.push(m_stk.top());
m_stk.pop();
}
}
private:
stack<int> m_stk;
stack<int> m_que;
};
/**
* 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();
*/