Implement Stack(lintcode 495)

Description

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

Example

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

Interface

class Stack {
    // Push a new item into the stack
    public void push(int x) {
        // Write your code here
    }

    // Pop the top of the stack
    public void pop() {
        // Write your code here
    }

    // Return the top of the stack
    public int top() {
        // Write your code here
    }

    // Check the stack is empty or not.
    public boolean isEmpty() {
        // Write your code here
    }    
}

Solution

class Stack {
    private ListNode head;
    // Push a new item into the stack
    public void push(int x) {
        // Write your code here
        ListNode newHead = new ListNode(x);
        newHead.next = head;
        head = newHead;
    }

    // Pop the top of the stack
    public void pop() {
        // Write your code here
        if (head == null) {
            throw new NoSuchElementException();
        }

        head = head.next;
    }

    // Return the top of the stack
    public int top() {
        // Write your code here
        if (head == null) {
            throw new NoSuchElementException();
        }
        return head.val;
    }

    // Check the stack is empty or not.
    public boolean isEmpty() {
        // Write your code here
        return head == null;
    }    
}