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225. Implement Stack using Queues
Total Accepted: 26676 Total Submissions: 88051 Difficulty: EasyImplement 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
, - 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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.
感觉没什么特别好的办法,反正两个队列,始终保持某一个为空,始终留一个(这个就是栈顶元素).....
申请两个队列1和2,举例,比如1,2,3,4依次压进队列1,则1应该是栈首,4是栈底
1,进栈:全部压进队列1
2,取栈顶元素:将队列1的元素转给队列2,队列1只剩1个即可,此元素就是栈首,最后再次压入队列2,使队列1为空,也就是说始终保持队列1或者2为空
3,弹出栈顶元素:如同取栈顶元素一样,留下的那一个弹出
4,是否为空?显然来个队列为空就是空栈。
class Stack {
public:
// Push element x onto stack.
void push(int x) {
if(!q1.empty())//q1不为空
q1.push(x);
else
q2.push(x);
newtop=x;//新进来的元素就是栈顶元素,pop时一定要更新这个值
}
// Removes the element on top of the stack.
void pop() {
if(!q1.empty())//q1不为空
{
while(q1.size()>1)
{
int val=q1.front();
if(q1.size() == 2)
newtop = val;
q2.push(val);
q1.pop();
}
q1.pop();
}
else
{
while(q2.size()>1)
{
int val=q2.front();
if(q2.size() == 2)
newtop = val;
q1.push(val);
q2.pop();
}
q2.pop();
}
}
// Get the top element.
int top() {
return newtop;
}
// Return whether the stack is empty.
bool empty() {
return q1.empty() && q2.empty();
}
private:
queue<int> q1;
queue<int> q2;
int newtop;
};
来自别人家的思路:
只申请一个队列,每次数据压进来的时候,直接就将数据调整为栈的顺序
将x压入队列,接着调整:取队首元素(先进来的元素)重新压倒队尾,操作que.size()-1次
牛逼.......
class Stack {
public:
queue<int> que;
// Push element x onto stack.
void push(int x) {
que.push(x);
for(int i=0;i<que.size()-1;++i){
que.push(que.front());
que.pop();
}
}
// Removes the element on top of the stack.
void pop() {
que.pop();
}
// Get the top element.
int top() {
return que.front();
}
// Return whether the stack is empty.
bool empty() {
return que.empty();
}
};
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原文地址:http://blog.csdn/ebowtang/article/details/50444995
原作者博客:http://blog.csdn/ebowtang
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