链表中倒数第k个节点

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.

方法一:两次遍历

class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }
}


public class Solution {

    public static void main(String[] args) {
        ListNode head = new ListNode(2);
        Solution solution = new Solution();
        ListNode kthFromEnd = solution.getKthFromEnd(head, 1);
        System.out.println(kthFromEnd.val);
    }


    public ListNode getKthFromEnd(ListNode head, int k) {
        int Len = 0;
        ListNode point = head;
        while (point != null) {
            Len += 1;
            point = point.next;
        }
        point = head;
        System.out.println("Len:" + Len);
        for (int i = 0; i < Len - k; i++) {
            point = point.next;
        }
        return point;
    }

}

方法二:双指针法

class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }
}


public class Solution {

    public static void main(String[] args) {
        ListNode head = new ListNode(2);
        Solution solution = new Solution();
        ListNode kthFromEnd = solution.getKthFromEnd(head, 1);
        System.out.println(kthFromEnd.val);
    }

public ListNode getKthFromEnd(ListNode head, int k) {
        ListNode start = head, end = head;
        for (int i = 0; i < k; i++) {
            end = end.next;
        }
        while (end != null) {
            end = end.next;
            start = start.next;
        }
        return start;
    }


}