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(0);
        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 -1;
        }
        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