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面试题 22. 链表中倒数第 k 个节点

题目描述

输入一个链表，输出该链表中倒数第 k 个节点。为了符合大多数人的习惯，本题从 1 开始计数，即链表的尾节点是倒数第 1 个节点。例如，一个链表有 6 个节点，从头节点开始，它们的值依次是 1、2、3、4、5、6。这个链表的倒数第 3 个节点是值为 4 的节点。

示例：

给定一个链表: 1->2->3->4->5, 和 k = 2.

返回链表 4->5.

解法

定义快慢指针 slow、fast，初始指向 head。

fast 先向前走 k 步，接着 slow、fast 同时向前走，当 fast 指向 null 时，slow 指向的节点即为链表的倒数第 k 个节点。

Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def getKthFromEnd(self, head: ListNode, k: int) -> ListNode:
        slow = fast = head
        for _ in range(k):
            fast = fast.next
        while fast:
            slow = slow.next
            fast = fast.next
        return slow

Java

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

JavaScript

/**
 * @param {ListNode} head
 * @param {number} k
 * @return {ListNode}
 */
var getKthFromEnd = function (head, k) {
    // 递归
    // let cnt = 1
    // function func(node) {
    //     if(!node || !node.next) return node
    //     let newNode = func(node.next)
    //     if(cnt === k) return newNode
    //     else cnt++
    //     return node
    // }
    // return func(head)

    // 快慢指针
    let slow = head;
    let fast = head;
    while (k) {
        fast = fast.next;
        k--;
    }
    while (fast) {
        slow = slow.next;
        fast = fast.next;
    }
    return slow;
};

Go

func getKthFromEnd(head *ListNode, k int) *ListNode {
    tmp := head
    for tmp != nil && k > 0{
        tmp = tmp.Next
        k--
    }
    slow := head
    fast := tmp
    for fast != nil {
        fast = fast.Next
        slow = slow.Next
    }
    return slow
}

C++

class Solution {
public:
    ListNode* getKthFromEnd(ListNode* head, int k) {
        ListNode *slow = head, *fast = head;
        while (k--) {
            fast = fast->next;
        }
        while (fast) {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};

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