在第i个结点前插入值为x的结点。 实验测试数据基本要求： 第一组数据：链表长度n≥10，x=100, i分别为5,n,n+1,0,1,n+2 第二组数据：链表长度n=0，x=100，i=5

好的，你需要实现一个在单向链表中在第i个结点前插入值为x的结点的方法。代码如下：

class ListNode {
    int val;
    ListNode next;

    ListNode(int val) {
        this.val = val;
        this.next = null;
    }
}

class Solution {
    public ListNode insert(ListNode head, int x, int i) {
        if (i < 1) {
            ListNode node = new ListNode(x);
            node.next = head;
            return node;
        }

        ListNode prev = null;
        ListNode curr = head;
        while (i > 1 && curr != null) {
            i--;
            prev = curr;
            curr = curr.next;
        }

        if (i > 1) {
            return head;
        }

        ListNode node = new ListNode(x);
        node.next = curr;
        if (prev != null) {
            prev.next = node;
        } else {
            head = node;
        }

        return head;
    }
}

public class Main {
    public static void main(String[] args) {
        ListNode head = null;
        int n = 10;
        for (int i = n; i >= 1; i--) {
            ListNode node = new ListNode(i);
            node.next = head;
            head = node;
        }

        Solution solution = new Solution();
        head = solution.insert(head, 100, 5);
        head = solution.insert(head, 100, n);
        head = solution.insert(head, 100, n + 1);
        head = solution.insert(head, 100, 0);
        head = solution.insert(head, 100, 1);
        head = solution.insert(head, 100, n + 2);

        ListNode node = head;
        while (node != null) {
            System.out.print(node.val + " ");
            node = node.next;
        }
        System.out.println();

        head = null;
        head = solution.insert(head, 100, 5);

        node = head;
        while (node != null) {
            System.out.print(node.val + " ");
            node = node.next;
        }
        System.out.println();
    }
}

第一组数据中，我们创建一个长度为10的单向链表，然后分别在第5个、第n个、第n+1个、第0个、第1个、第n+2个结点前插入值为100的结点。第二组数据中，我们创建一个空链表，然后在第5个结点前插入值为100的结点。