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1721. 交换链表中的节点

题目描述

给你链表的头节点 head 和一个整数 k 。

交换 链表正数第 k 个节点和倒数第 k 个节点的值后，返回链表的头节点（链表 从 1 开始索引）。

 

示例 1：

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

示例 2：

输入：head = [7,9,6,6,7,8,3,0,9,5], k = 5
输出：[7,9,6,6,8,7,3,0,9,5]

示例 3：

输入：head = [1], k = 1
输出：[1]

示例 4：

输入：head = [1,2], k = 1
输出：[2,1]

示例 5：

输入：head = [1,2,3], k = 2
输出：[1,2,3]

 

提示：

  • 链表中节点的数目是 n
  • 1 <= k <= n <= 105
  • 0 <= Node.val <= 100

解法

方法一：快慢指针

我们可以先用快指针 $fast$ 找到链表的第 $k$ 个节点，用指针 $p$ 指向它。然后我们再用慢指针 $slow$ 从链表的头节点出发，快慢指针同时向后移动，当快指针到达链表的最后一个节点时，慢指针 $slow$ 恰好指向倒数第 $k$ 个节点，用指针 $q$ 指向它。此时，我们只需要交换 $p$ 和 $q$ 的值即可。

时间复杂度 $O(n)$，其中 $n$ 是链表的长度。空间复杂度 $O(1)$。

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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def swapNodes(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
        fast = slow = head
        for _ in range(k - 1):
            fast = fast.next
        p = fast
        while fast.next:
            fast, slow = fast.next, slow.next
        q = slow
        p.val, q.val = q.val, p.val
        return head

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/**
 * 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 swapNodes(ListNode head, int k) {
        ListNode fast = head;
        while (--k > 0) {
            fast = fast.next;
        }
        ListNode p = fast;
        ListNode slow = head;
        while (fast.next != null) {
            fast = fast.next;
            slow = slow.next;
        }
        ListNode q = slow;
        int t = p.val;
        p.val = q.val;
        q.val = t;
        return head;
    }
}

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* swapNodes(ListNode* head, int k) {
        ListNode* fast = head;
        while (--k) {
            fast = fast->next;
        }
        ListNode* slow = head;
        ListNode* p = fast;
        while (fast->next) {
            fast = fast->next;
            slow = slow->next;
        }
        ListNode* q = slow;
        swap(p->val, q->val);
        return head;
    }
};

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/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func swapNodes(head *ListNode, k int) *ListNode {
    fast := head
    for ; k > 1; k-- {
        fast = fast.Next
    }
    p := fast
    slow := head
    for fast.Next != nil {
        fast, slow = fast.Next, slow.Next
    }
    q := slow
    p.Val, q.Val = q.Val, p.Val
    return head
}

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/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function swapNodes(head: ListNode | null, k: number): ListNode | null {
    let [fast, slow] = [head, head];
    while (--k) {
        fast = fast.next;
    }
    const p = fast;
    while (fast.next) {
        fast = fast.next;
        slow = slow.next;
    }
    const q = slow;
    [p.val, q.val] = [q.val, p.val];
    return head;
}

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode SwapNodes(ListNode head, int k) {
        ListNode fast = head;
        while (--k > 0) {
            fast = fast.next;
        }
        ListNode p = fast;
        ListNode slow = head;
        while (fast.next != null) {
            fast = fast.next;
            slow = slow.next;
        }
        ListNode q = slow;
        int t = p.val;
        p.val = q.val;
        q.val = t;
        return head;
    }
}

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