Implement Queue using Stacks

问题：

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

, and

is 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).

解决：

【注】关于测试参数的解释：

["MyQueue","push","push","peek","pop","pop","empty"] --- 表示要进行的操作

[[],[1],[2],[],[],[],[]] --- 表示对应传入的参数

① 使用两个栈s1，s2，进栈的时候不做任何处理，出栈的时候把栈逆序放在另外一个栈，出另外一个栈。
之前写经常出现空栈异常或者超时，就是没有写好s1，s2之间的转换。

class MyQueue { // 87ms
Stack<Integer> s1 = new Stack<>();
Stack<Integer> s2 = new Stack<>();
/** Initialize your data structure here. */
public MyQueue() {}
/** Push element x to the back of queue. */
public void push(int x) {
while(! s2.isEmpty()) s1.push(s2.pop());
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
while(! s1.isEmpty()) s2.push(s1.pop());
return s2.pop();
}
/** Get the front element. */
public int peek() {
while(! s1.isEmpty()) s2.push(s1.pop());
return s2.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
while(! s2.isEmpty()) s1.push(s2.pop());
return s1.isEmpty();
}
}
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* boolean param_4 = obj.empty();
*/

② 使用两个栈，在进栈的时候，把栈逆序放在另外一个栈，出的时候直接出另外一个栈就可以了。

class MyQueue { //93ms
Stack<Integer> s1 = new Stack<>();
Stack<Integer> s2 = new Stack<>();
/** Initialize your data structure here. */
public MyQueue() {}
/** Push element x to the back of queue. */
public void push(int x) {
while(! s1.isEmpty()) s2.push(s1.pop());
s2.push(x);
while(! s2.isEmpty()) s1.push(s2.pop());
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
return s1.pop();
}
/** Get the front element. */
public int peek() {
return s1.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return s1.isEmpty();
}
}

③ 除了使用两个栈之外，额外使用一个指针指向头部。

class MyQueue { //113ms
Stack<Integer> s1 = new Stack<>();
Stack<Integer> s2 = new Stack<>();
int head;
/** Initialize your data structure here. */
public MyQueue() {}
/** Push element x to the back of queue. */
public void push(int x) {
if(s1.isEmpty()) head = x;
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
while(! s1.isEmpty()) s2.push(s1.pop());
int top = -1;
if(! s2.isEmpty()){
top = s2.pop();
if(! s2.isEmpty()) head = s2.peek();
}
while(! s2.isEmpty()) s1.push(s2.pop());
return top;
}
/** Get the front element. */
public int peek() {
return head;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return s1.isEmpty();
}
}