[225]Implement Stack using Queues

【题目描述】

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.
Notes:
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.
【思路】

queue成员函数

empty()判断队列空，当队列空时，返回true。

size()访问队列中的元素个数。

push()会将一个元素置入queue中。

front()会返回queue内的第一个元素(也就是第一个被置入的元素)。

back()会返回queue中最后一个元素(也就是最后被插入的元素)。

pop()会从queue中移除一个元素。[1] 

注意：pop()虽然会移除下一个元素，但是并不返回它，front()和back()返回下一个元素但并不移除该元素。

【代码】

class Stack {
public:
queue<int> s;
// Push element x onto stack.
void push(int x) {
s.push(x);
}

// Removes the element on top of the stack.
void pop() {
int n=s.size();
if(n==0) return;
for(int i=0;i<n-1;i++){
s.push(s.front());
s.pop();
}
s.pop();
}

// Get the top element.
int top() {
return s.back();
}

// Return whether the stack is empty.
bool empty() {
return s.empty();
}
};

update:

用两个queue来实现了栈，原理基本相同，不过在实现的时候要注意判断是哪个queue为空，对另一个queue操作：

class Stack {
public:
queue<int>q1,q2;
// Push element x onto stack.
void push(int x) {
if(q2.size()>0) q2.push(x);
else q1.push(x);
}

// Removes the element on top of the stack.
void pop() {
int data;
if(q1.size()<=0){
while(q2.size()>1){
data=q2.front();
q2.pop();
q1.push(data);
}
q2.pop();
}
else if(q2.size()<=0){
while(q1.size()>1){
data=q1.front();
q1.pop();
q2.push(data);
}
q1.pop();
}
}

// Get the top element.
int top() {
int data;
if(q1.size()<=0){
return q2.back();
}
if(q2.size()<=0){
return q1.back();
}
}

// Return whether the stack is empty.
bool empty() {
if(q1.size()<=0&&q2.size()<=0) return true;
else return false;
}
};