19. Remove Nth Node From End of List

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LeetCode

Two Pass

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/**
 * 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(-1);
        dummy.next=head;
        ListNode start=head;
        int length=0;
        while(start!=null){
            length++;
            start=start.next;
        }
        int pos=length-n;
        start=dummy;
        while(pos>0){            
            pos--;
            start=start.next;
        }
        start.next=start.next.next;

        return dummy.next;
    }
}


One Pass

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/**
 * 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 first = dummy;
        ListNode second = dummy;
        // Advances first pointer so that the gap between first and second is n nodes apart
        for (int i = 0; i <= n; i++) {
            first = first.next;
        }
        // Move first to the end, maintaining the gap
        while (first != null) {
            first = first.next;
            second = second.next;
        }
        second.next = second.next.next;
        return dummy.next;
    }
}

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