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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.
- 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).
解题:
用栈实现队列,这个比用队列实现栈要麻烦一些,这里用到两个栈,两种思路,第一种就是在进栈的时候,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。第二种思路,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。下面就是两种的代码实现。
在进栈的时候进行处理:
class MyQueue {
// Push element x to the back of queue.
Stack<Integer> stack=new Stack<>();
Stack<Integer> stack2=new Stack<>();
public void push(int x) {
while(!stack.isEmpty())
{
stack2.push(stack.pop());
}
stack2.push(x);
while(!stack2.isEmpty())
{
stack.push(stack2.pop());
}
}
// Removes the element from in front of queue.
public void pop() {
stack.pop();
}
// Get the front element.
public int peek() {
return stack.peek();
}
// Return whether the queue is empty.
public boolean empty() {
return stack.isEmpty();
}
}
在出栈的时候进行处理:
class MyQueue2 {
// Push element x to the back of queue.
Stack<Integer> stack=new Stack<>();
Stack<Integer> stack2=new Stack<>();
public void push(int x) {
while(!stack2.isEmpty())
stack.push(stack2.pop());
stack.push(x);
}
// Removes the element from in front of queue.
public void pop() {
while(!stack.isEmpty())
stack2.push(stack.pop());
stack2.pop();
}
// Get the front element.
public int peek() {
while(!stack.isEmpty())
stack2.push(stack.pop());
return stack2.peek();
}
// Return whether the queue is empty.
public boolean empty() {
while(!stack2.isEmpty())
stack.push(stack2.pop());
return stack.isEmpty();
}
}
本文标签: 题解implementQueueJavaStacks
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