225. 用队列实现栈
使用队列实现栈的下列操作：
- push(x) -- 元素 x 入栈
- pop() -- 移除栈顶元素
- top() -- 获取栈顶元素
- empty() -- 返回栈是否为空
注意:
- 你只能使用队列的基本操作-- 也就是
push to back,peek/pop from front,size, 和is empty这些操作是合法的。 - 你所使用的语言也许不支持队列。 你可以使用 list 或者 deque（双端队列）来模拟一个队列 , 只要是标准的队列操作即可。
- 你可以假设所有操作都是有效的（例如, 对一个空的栈不会调用 pop 或者 top 操作）。
解法一
用两个队列模拟栈
入队 O(1) 出队 O(n)
class MyStack {
private static Queue<Integer> q1;
private static Queue<Integer> q2;
/** Initialize your data structure here. */
public MyStack() {
q1 = new LinkedList<>();
q2 = new LinkedList<>();
}
/** Push element x onto stack. */
public void push(int x) {
q1.offer(x);
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
while(q1.size() > 1) {
q2.offer(q1.poll());
}
int result = q1.poll();
Queue tmp = q1;
q1 = q2;
q2 = tmp;
return result;
}
/** Get the top element. */
public int top() {
while(q1.size() > 1) {
q2.offer(q1.poll());
}
int result = q1.peek();
q2.offer(q1.poll());
Queue tmp = q1;
q1 = q2;
q2 = tmp;
return result;
}
/** Returns whether the stack is empty. */
public boolean empty() {
return q1.isEmpty() && q2.isEmpty();
}
}
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/
优化
用两个队列模拟栈 增加一个 top
class MyStack {
private static Queue<Integer> q1;
private static Queue<Integer> q2;
private int top;
/** Initialize your data structure here. */
public MyStack() {
q1 = new LinkedList<>();
q2 = new LinkedList<>();
}
/** Push element x onto stack. */
public void push(int x) {
q1.offer(x);
top = x;
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
while(q1.size() > 1) {
top = q1.poll();
q2.offer(top);
}
int result = q1.poll();
Queue tmp = q1;
q1 = q2;
q2 = tmp;
return result;
}
/** Get the top element. */
public int top() {
return top;
}
/** Returns whether the stack is empty. */
public boolean empty() {
return q1.isEmpty() && q2.isEmpty();
}
}
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/
解法二
用两个队列模拟栈
入队 O(1)
出队 O(n)
class MyStack {
private static Queue<Integer> q1;
private static Queue<Integer> q2;
private int top;
/** Initialize your data structure here. */
public MyStack() {
q1 = new LinkedList<>();
q2 = new LinkedList<>();
}
/** Push element x onto stack. */
public void push(int x) {
q2.offer(x);
while(q1.size() > 0) q2.offer(q1.poll());
Queue<Integer> tmp = q1;
q1 = q2;
q2 = tmp;
top = x;
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
return q1.poll();
}
/** Get the top element. */
public int top() {
return q1.peek();
}
/** Returns whether the stack is empty. */
public boolean empty() {
return q1.isEmpty() && q2.isEmpty();
}
}
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/
解法三
用一个队列
入队：O(n)
出队：O(1)
class MyStack {
private static Queue<Integer> queue;
/** Initialize your data structure here. */
public MyStack() {
queue = new LinkedList<>();
}
/** Push element x onto stack. */
public void push(int x) {
queue.offer(x);
int size = queue.size();
while(size > 1) {
queue.offer(queue.poll());
size--;
};
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
return queue.poll();
}
/** Get the top element. */
public int top() {
return queue.peek();
}
/** Returns whether the stack is empty. */
public boolean empty() {
return queue.isEmpty();
}
}
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/