495. Implement Stack【easy】

Implement a stack. You can use any data structure inside a stack except stack itself to implement it.

Have you met this question in a real interview?

 

 

Example

push(1)
pop()
push(2)
top()  // return 2
pop()
isEmpty() // return true
push(3)
isEmpty() // return false

解法一：

 class Stack {
 public:
     struct Node {
         int val;
         Node *prev, *next;
         Node(int v) {
             val = v;
             prev = NULL;
             next = NULL;
         }
     };

     Node *dummy;
     Node *tail;
     Stack() {
         dummy = new Node();
         tail = dummy;
     }

     ~Stack() {

     }

     void push(int x) {
         tail->next = new Node(x);
         tail->next->prev = tail;
         tail = tail->next;
     }

     // Pop the top of the stack
     void pop() {
         if (dummy->next == NULL) {
             return;
         }
         Node *prev = tail->prev;
         prev->next = NULL;
         tail = prev;
     }

     // Return the top of the stack
     int top() {
         if (dummy->next == NULL) {
             return -;
         }
         return tail->val;
     }

     // Check the stack is empty or not.
     bool isEmpty() {
         if (dummy->next == NULL) {
             return true;
         }
         return false;
     }
 };

用链表实现栈，因为栈pop出的是最后一个，如果用尾插法的单向链表明显不太方便，所以用双向链表。

参考@YI 的代码

https://yixuanwangblog.wordpress.com/2016/09/04/lintcode-495-implement-stack/

解法二：

 class ListNode{
     int val;
     ListNode next;
     public ListNode(int val) {
         this.val = val;
     }
 };

 public class Stack {
     ListNode head;
     public Stack(){
        head = new ListNode(0);
     }

     /*
      * @param x: An integer
      * @return: nothing
      */
     public void push(int x) {
         ListNode node = new ListNode(x);
         node.next = head.next;
         head.next = node;
     }

     /*
      * @return: nothing
      */
     public int pop() {
         ListNode top = head.next;
         head.next = head.next.next;
         return top.val;
     }

     /*
      * @return: An integer
      */
     public int top() {
         return head.next.val;
     }

     /*
      * @return: True if the stack is empty
      */
     public boolean isEmpty() {
         if (head.next == null) {
             return true;
         } else{
             return false;
         }
     }
 }

用链表实现栈，使用头插法。

参考@zhengyang2015 的代码

https://zhengyang2015.gitbooks.io/lintcode/implement_stack_495.html