019 Remove Nth Node From End of List.py

'''
Given a linked list, remove the nth node from the end of list and return its head.

For example,

   Given linked list: 1->2->3->4->5, and n = 2.

   After removing the second node from the end, the linked list becomes 1->2->3->5.
Note:
Given n will always be valid.
Try to do this in one pass.
'''

# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

    # Define this to check if it works well
    def myPrint(self):
        print(self.val)
        if self.next:
            self.next.myPrint()


class Solution(object):
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        if not head:
            return head
        dummy = ListNode(-1)
        dummy.next=head
        prev = dummy
        cur = dummy
        while prev and n >= 0:
            prev = prev.next
            n -= 1
        while prev:
            prev = prev.next
            cur = cur.next
        cur.next = cur.next.next
        return dummy.next


if __name__ == "__main__":
    n5 = ListNode(5)
    n4 = ListNode(4)
    n3 = ListNode(3)
    n2 = ListNode(2)
    n1 = ListNode(1)
    n1.next = n2
    n2.next = n3
    n3.next = n4
    n4.next = n5
    result = Solution().removeNthFromEnd(n1, 5)
    result.myPrint()