RemoveNthNodeFromEnd.java

package com.freetymekiyan.algorithms.level.easy;

import com.freetymekiyan.algorithms.utils.Utils.ListNode;

/**
 * Given a linked list, remove the nth node from the end of list and return its
 * head.
 * <p>
 * For example,
 * <p>
 * Given linked list: 1->2->3->4->5, and n = 2.
 * <p>
 * After removing the second node from the end, the linked list becomes
 * 1->2->3->5.
 * <p>
 * Note:
 * Given n will always be valid.
 * Try to do this in one pass.
 * <p>
 * Tags: Linked list, Two pointers
 */
class RemoveNthNodeFromEnd {

    /**
     * Dummy head and Runner's technique
     */
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode pre = new ListNode(0);
        pre.next = head;
        ListNode p1 = pre;
        ListNode p2 = pre;
        int i = 0;
        while (i < n) {
            p2 = p2.next;
            i++;
        }
        while (p2.next != null) {
            p1 = p1.next;
            p2 = p2.next;
        }

        p1.next = p1.next.next;
        return pre.next;
    }
}