Merge pull request #2518 from PHJ369906/beChild-bugfix-branch

Update: 修复原Java版本报空指针异常(NullPointerException)

problems/0019.删除链表的倒数第N个节点.md

@@ -98,27 +98,33 @@ public:
 ### Java:
 
 ```java
-public ListNode removeNthFromEnd(ListNode head, int n){
-    ListNode dummyNode = new ListNode(0);
-    dummyNode.next = head;
+class Solution {
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        //新建一个虚拟头节点指向head
+        ListNode dummyNode = new ListNode(0);
+        dummyNode.next = head;
+        //快慢指针指向虚拟头节点
+        ListNode fastIndex = dummyNode;
+        ListNode slowIndex = dummyNode;
 
-    ListNode fastIndex = dummyNode;
-    ListNode slowIndex = dummyNode;
+        // 只要快慢指针相差 n 个结点即可
+        for (int i = 0; i <= n; i++) {
+            fastIndex = fastIndex.next;
+        }
 
-    // 只要快慢指针相差 n 个结点即可
-    for (int i = 0; i < n  ; i++){ 
-        fastIndex = fastIndex.next;
+        while (fastIndex != null) {
+            fastIndex = fastIndex.next;
+            slowIndex = slowIndex.next;
+        }
+
+        // 此时 slowIndex 的位置就是待删除元素的前一个位置。
+        // 具体情况可自己画一个链表长度为 3 的图来模拟代码来理解
+        // 检查 slowIndex.next 是否为 null，以避免空指针异常
+        if (slowIndex.next != null) {
+            slowIndex.next = slowIndex.next.next;
+        }
+        return dummyNode.next;
     }
-
-    while (fastIndex != null){
-        fastIndex = fastIndex.next;
-        slowIndex = slowIndex.next;
-    }
-
-    //此时 slowIndex 的位置就是待删除元素的前一个位置。
-    //具体情况可自己画一个链表长度为 3 的图来模拟代码来理解
-    slowIndex.next = slowIndex.next.next;
-    return dummyNode.next;
 }
 ```