19_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

class Solution(object):
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        l, r = head, head
        while n > 0:
            r = r.next
            n -= 1
        if r == None:
            head = head.next
            return head
        while r.next != None:
            l = l.next
            r = r.next
        l.next = l.next.next
        return head