lc-19

//Given the head of a linked list, remove the nth node from the end of the list 
//and return its head. 
//
// 
// Example 1: 
//
// 
//Input: head = [1,2,3,4,5], n = 2
//Output: [1,2,3,5]
// 
//
// Example 2: 
//
// 
//Input: head = [1], n = 1
//Output: []
// 
//
// Example 3: 
//
// 
//Input: head = [1,2], n = 1
//Output: [1]
// 
//
// 
// Constraints: 
//
// 
// The number of nodes in the list is sz. 
// 1 <= sz <= 30 
// 0 <= Node.val <= 100 
// 1 <= n <= sz 
// 
//
// 
// Follow up: Could you do this in one pass? 
// Related Topics Linked List Two Pointers 
// 👍 10475 👎 494


//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        int listLen = 0;
        ListNode temp = head;
        
        if (head == null || n <= 0) {
            return head;
        }
        
        while (temp != null) {
            listLen++;
            temp = temp.next;
        }
        
        if (listLen < n) {
            return head;
        }
        if (listLen == n) {
            ListNode result = head.next;
            head.next = null;
            return result;
        }
        
        int subLen = listLen - n - 1;
        temp = head;
        for (int i = 0; i < subLen; i++) {
            temp = temp.next;
        }
        
        ListNode removeNode = temp.next;
        temp.next = removeNode.next;
        removeNode.next = null;
        return head;
    }
}
//leetcode submit region end(Prohibit modification and deletion)