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## 19.删除链表的倒数第N个节点

[力扣题目链接](https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/)

给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点。

进阶：你能尝试使用一趟扫描实现吗？

示例 1：

![19.删除链表的倒数第N个节点](https://img-blog.csdnimg.cn/20210510085957392.png)

输入：head = [1,2,3,4,5], n = 2
输出：[1,2,3,5]
示例 2：

输入：head = [1], n = 1
输出：[]
示例 3：

输入：head = [1,2], n = 1
输出：[1]


## 思路

双指针的经典应用，如果要删除倒数第n个节点，让fast移动n步，然后让fast和slow同时移动，直到fast指向链表末尾。删掉slow所指向的节点就可以了。

思路是这样的，但要注意一些细节。

分为如下几步：

* 首先这里我推荐大家使用虚拟头结点，这样方面处理删除实际头结点的逻辑，如果虚拟头结点不清楚，可以看这篇： [链表：听说用虚拟头节点会方便很多？](https://programmercarl.com/0203.移除链表元素.html)

* 定义fast指针和slow指针，初始值为虚拟头结点，如图：

<img src='https://code-thinking.cdn.bcebos.com/pics/19.%E5%88%A0%E9%99%A4%E9%93%BE%E8%A1%A8%E7%9A%84%E5%80%92%E6%95%B0%E7%AC%ACN%E4%B8%AA%E8%8A%82%E7%82%B9.png' width=600> </img></div>

* fast首先走n + 1步 ，为什么是n+1呢，因为只有这样同时移动的时候slow才能指向删除节点的上一个节点（方便做删除操作），如图：
<img src='https://code-thinking.cdn.bcebos.com/pics/19.%E5%88%A0%E9%99%A4%E9%93%BE%E8%A1%A8%E7%9A%84%E5%80%92%E6%95%B0%E7%AC%ACN%E4%B8%AA%E8%8A%82%E7%82%B91.png' width=600> </img></div>

* fast和slow同时移动，直到fast指向末尾，如题：
<img src='https://code-thinking.cdn.bcebos.com/pics/19.%E5%88%A0%E9%99%A4%E9%93%BE%E8%A1%A8%E7%9A%84%E5%80%92%E6%95%B0%E7%AC%ACN%E4%B8%AA%E8%8A%82%E7%82%B92.png' width=600> </img></div>

* 删除slow指向的下一个节点，如图：
<img src='https://code-thinking.cdn.bcebos.com/pics/19.%E5%88%A0%E9%99%A4%E9%93%BE%E8%A1%A8%E7%9A%84%E5%80%92%E6%95%B0%E7%AC%ACN%E4%B8%AA%E8%8A%82%E7%82%B93.png' width=600> </img></div>

此时不难写出如下C++代码：

```CPP
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* dummyHead = new ListNode(0);
        dummyHead->next = head;
        ListNode* slow = dummyHead;
        ListNode* fast = dummyHead;
        while(n-- && fast != NULL) {
            fast = fast->next;
        }
        fast = fast->next; // fast再提前走一步，因为需要让slow指向删除节点的上一个节点
        while (fast != NULL) {
            fast = fast->next;
            slow = slow->next;
        }
        slow->next = slow->next->next;
        return dummyHead->next;
    }
};
```


## 其他语言版本

java:

```java
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;

        ListNode slow = dummy;
        ListNode fast = dummy;
        while (n-- > 0) {
            fast = fast.next;
        }
        // 记住 待删除节点slow 的上一节点
        ListNode prev = null;
        while (fast != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next;
        }
        // 上一节点的next指针绕过 待删除节点slow 直接指向slow的下一节点
        prev.next = slow.next;
        // 释放 待删除节点slow 的next指针, 这句删掉也能AC
        slow.next = null;

        return dummy.next;
    }
}
```

Python:
```python
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        head_dummy = ListNode()
        head_dummy.next = head

        slow, fast = head_dummy, head_dummy
        while(n!=0): #fast先往前走n步
            fast = fast.next
            n -= 1
        while(fast.next!=None):
            slow = slow.next
            fast = fast.next
        #fast 走到结尾后，slow的下一个节点为倒数第N个节点
        slow.next = slow.next.next #删除
        return head_dummy.next
```
Go:
```Go
/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func removeNthFromEnd(head *ListNode, n int) *ListNode {
    dummyHead := &ListNode{}
    dummyHead.Next = head
    cur := head
    prev := dummyHead
    i := 1
    for cur != nil {
        cur = cur.Next
        if i > n {
            prev = prev.Next
        }
        i++
    }
    prev.Next = prev.Next.Next
    return dummyHead.Next
}
```

JavaScript:

```js
/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
var removeNthFromEnd = function(head, n) {
    let ret = new ListNode(0, head),
        slow = fast = ret;
    while(n--) fast = fast.next;
    while (fast.next !== null) {
        fast = fast.next; 
        slow = slow.next
    };
    slow.next = slow.next.next;
    return ret.next;
};
```
TypeScript:

版本一（快慢指针法）：

```typescript
function removeNthFromEnd(head: ListNode | null, n: number): ListNode | null {
    let newHead: ListNode | null = new ListNode(0, head);
    let slowNode: ListNode | null = newHead,
        fastNode: ListNode | null = newHead;
    for (let i = 0; i < n; i++) {
        fastNode = fastNode.next;
    }
    while (fastNode.next) {
        fastNode = fastNode.next;
        slowNode = slowNode.next;
    }
    slowNode.next = slowNode.next.next;
    return newHead.next;
};
```

版本二（计算节点总数法）：

```typescript
function removeNthFromEnd(head: ListNode | null, n: number): ListNode | null {
    let curNode: ListNode | null = head;
    let listSize: number = 0;
    while (curNode) {
        curNode = curNode.next;
        listSize++;
    }
    if (listSize === n) {
        head = head.next;
    } else {
        curNode = head;
        for (let i = 0; i < listSize - n - 1; i++) {
            curNode = curNode.next;
        }
        curNode.next = curNode.next.next;
    }
    return head;
};
```

版本三（递归倒退n法）：

```typescript
function removeNthFromEnd(head: ListNode | null, n: number): ListNode | null {
    let newHead: ListNode | null = new ListNode(0, head);
    let cnt = 0;
    function recur(node) {
        if (node === null) return;
        recur(node.next);
        cnt++;
        if (cnt === n + 1) {
            node.next = node.next.next;
        }
    }
    recur(newHead);
    return newHead.next;
};
```

Kotlin:

```Kotlin
fun removeNthFromEnd(head: ListNode?, n: Int): ListNode? {
    val pre = ListNode(0).apply {
        this.next = head
    }
    var fastNode: ListNode? = pre
    var slowNode: ListNode? = pre
    for (i in 0..n) {
        fastNode = fastNode?.next
    }
    while (fastNode != null) {
        slowNode = slowNode?.next
        fastNode = fastNode.next
    }
    slowNode?.next = slowNode?.next?.next
    return pre.next
}
```

Swift：
```swift
func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
    if head == nil {
        return nil
    }
    if n == 0 {
        return head
    }
    let dummyHead = ListNode(-1, head)
    var fast: ListNode? = dummyHead
    var slow: ListNode? = dummyHead
    // fast 前移 n
    for _ in 0 ..< n {
        fast = fast?.next
    }
    while fast?.next != nil {
        fast = fast?.next
        slow = slow?.next
    }
    slow?.next = slow?.next?.next
    return dummyHead.next
}
```

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