使用栈实现队列的下列操作:
push(x) -- 将一个元素放入队列的尾部。
pop() -- 从队列首部移除元素。
peek() -- 返回队列首部的元素。
empty() -- 返回队列是否为空。
示例:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // 返回 1
queue.pop(); // 返回 1
queue.empty(); // 返回 false
说明:
你只能使用标准的栈操作 – 也就是只有 push to top, peek/pop from top, size, 和 is empty
操作是合法的。 你所使用的语言也许不支持栈。你可以使用 list 或者 deque(双端队列)来模拟一个栈,只要是标准的栈操作即可。
假设所有操作都是有效的 (例如,一个空的队列不会调用 pop 或者 peek 操作)。来源:力扣(LeetCode)
链接:https://leetcode-cn.com/problems/implement-queue-using-stacks
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昨天学习了用队列实现栈的操作,一样的照葫芦画瓢,第一次写的代码如下:
class MyQueue {
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) {
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
while(s1.size()>1){
s2.push(s1.pop());
}
int p = s1.pop();
Stack<Integer> tem = s1;
s1=s2;
s2=tem;
return p;
}
/** Get the front element. */
public int peek() {
while(s1.size()>1){
s2.push(s1.pop());
}
int p = s1.pop();
Stack<Integer> tem = s1;
s2.push(p);
s1=s2;
s2=tem;
return p;
}
/** Returns whether the queue is empty. */
public boolean empty() {
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();
*/
可惜不对,分析原因,在s2.push(s1.pop()),s1=s2的时候实际上是将s1给反转了,所以对于push操作1,2,3,在pop后s1就变为2,1了,所以修改代码如下:
class MyQueue {
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) {
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
while(s1.size()>1){
s2.push(s1.pop());
}
int p = s1.pop();
// Stack<Integer> tem = s1;
// s1=s2;
// s2=tem;
while(s2.size()>0){
s1.push(s2.pop());
}
return p;
}
/** Get the front element. */
public int peek() {
while(s1.size()>1){
s2.push(s1.pop());
}
int p = s1.pop();
s2.push(p);
while(s2.size()>0){
s1.push(s2.pop());
}
return p;
}
/** Returns whether the queue is empty. */
public boolean empty() {
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();
*/
通过,正统的写法还是看官方题解吧