LeetCode232 Implement Queue using Stacks Java 題解

標籤：leetcode   java   implement queue   using stacks   

題目：

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.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is emptyoperations 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).

解題：

用棧實現隊列，這個比用隊列實現棧要麻煩一些，這裡用到兩個棧，兩種思路，第一種就是在進棧的時候，把棧逆序放在另外一個棧，出的時候直接出另外一個棧就可以了。第二種思路，進棧的時候不做任何處理，出棧的時候把棧逆序放在另外一個棧，出另外一個棧。下面就是兩種的代碼實現。

在進棧的時候進行處理：

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();    }}

在出棧的時候進行處理：

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();    }}

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LeetCode232 Implement Queue using Stacks Java 題解