题目: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.
Notes:
- You must use only standard operations of a stack -- which means only
push to top
,peek/pop from top
,size
, andis 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).
题目解答:
题目中的要求是使用栈来实现一个队列的功能,具体思路很简单,使用到了两个栈来模拟一个队列的功能。
代码如下:
class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
s1.push(x);
if(s2.empty())
{
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
}
}
// Removes the element from in front of queue.
void pop(void) {
if(!s2.empty())
{
s2.pop();
}
else if(!s1.empty())
{
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
s2.pop();
}
}
// Get the front element.
int peek(void) {
if(!s2.empty())
{
return s2.top();
}
else if(!s1.empty())
{
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
return s2.top();
}
return -1;
}
// Return whether the queue is empty.
bool empty(void) {
if( (s1.empty()) && (s2.empty()) )
return true;
else
return false;
}
private:
stack<int> s1;
stack<int> s2;
};