19 Remove Nth Node From End of List

19. Remove Nth Node From End of List

1. Question

Given a linked list, remove thenthnode 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.

2. Implementation

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;

        ListNode fast = dummy, slow = dummy;

        for (int i = 1; i <= n + 1; i++) {
            fast = fast.next;
        }

        while (fast != null) {
            fast = fast.next;
            slow = slow.next;
        }

        slow.next = slow.next.next;
        return dummy.next;
    }
}

3. Time & Space Complexity

时间复杂度O(n), 空间复杂度O(1)