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题目链接:https://leetcode/problems/implement-stack-using-queues/
题目: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.
- You must use only standard operations of a queue -- which means only
push to back
,peek/pop from front
,size
, andis 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).
public class Solution
{
Queue<Integer> q1=new ArrayDeque<>();
Queue<Integer> q2=new ArrayDeque<>();
// Push element x onto stack.
public void push(int x)
{
if(q1.isEmpty())
{
q1.add(x);
while(!q2.isEmpty())
{
q1.add(q2.poll());
}
}
else
{
q2.add(x);
while(!q1.isEmpty())
{
q2.add(q1.poll());
}
}
}
// Removes the element on top of the stack.
public void pop()
{
if(!q1.isEmpty())
q1.poll();
if(!q2.isEmpty())
q2.poll();
}
// Get the top element.
public int top()
{
if (!q1.isEmpty())
return q1.peek();
if (!q2.isEmpty())
return q2.peek();
else
return -1;
}
// Return whether the stack is empty.
public boolean empty()
{
return q1.isEmpty() && q2.isEmpty();
}
}
本文标签: OJLeetCodequeuesStackimplement
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