Remove Nth Node From End of List

removeNthFromEnd.java

/*
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.

Algorithm:
1. 3 pointers, last, mid and ahead, the distance between mid and ahead is n and last is just behind the mid
2. It needs dummy node to handle corner case.
*/
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(head == null) return null;
        ListNode ahead = head;
        ListNode mid = head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode last = dummy;
        
        while(n > 0 && ahead != null) {
            ahead = ahead.next;
            n--;
        }
        
        while(ahead != null) {
            ahead = ahead.next;
            last = mid;
            mid = mid.next;
        }
        
        // delete mid
        last.next = mid.next;
        return dummy.next;
    }
}