输出链表的倒数第N个元素-Java

思路，倒数第N个元素，遍历链表，构造两个指针。

第一个从头先遍历N个节点，然后停下来。

第一个接着继续遍历，第二个从头开始遍历(同时遍历)，直到第一个遍历到null为止。

这时第二个指针所在的位置就是倒数第N个元素的位置。

public class NumKNumber {
    public static class ListNode{
        ListNode next = null;
        int data;
        public ListNode(int data){
            this.data = data;
        }
    } 
    public static void main(String[] args) {
        NumKNumber numKNumber = new NumKNumber();
        ListNode l1 = new ListNode(1);
        ListNode l2 = new ListNode(2);
        ListNode l3 = new ListNode(3);
        ListNode l4 = new ListNode(4);
        ListNode l5 = new ListNode(5);
        ListNode l6 = new ListNode(6);
        l1.next = l2;
        l2.next = l3;
        l3.next = l4;
        l4.next = l5;
        l5.next = l6;
        int k = 10;
        ListNode res = numKNumber.getNumKNumber(l1, k);
        if(res != null){
            System.out.println(res.data);
        }else{
            System.out.println("异常");
        }
        
    }
    
    public ListNode getNumKNumber(ListNode head, int k){
        if(k < 0 || head == null){
            return null;
        }

        ListNode curr = head;
        int len = 0;
        // 判k是否大于链表长度
        while(curr != null){
            curr = curr.next;
            len++;
        }
        if(len < k){
            return null;
        }
        curr = head; //curr复位
        //向后移动k个
        while(k != 0){
            curr = curr.next;
            k--;
        }
        ListNode nodeK = head;
        while(curr != null){
            curr = curr.next;
            nodeK = nodeK.next;
        }
        return nodeK;


    }
}

下面这个没有判断k的取值，以及链表是否为空。

public class BackNNode {
    public static class ListNode {
        int val;
        ListNode next = null;
    
        ListNode(int val) {
            this.val = val;
        }
    }
    public static void main(String[] args) {
        ListNode l1 = new ListNode(1);
        ListNode l2 = new ListNode(2);
        ListNode l3 = new ListNode(3);
        ListNode l4 = new ListNode(3);
        ListNode l5 = new ListNode(4);
        ListNode l6 = new ListNode(4);
        l1.next = l2;
        l2.next = l3;
        l3.next = l4;
        l4.next = l5;
        l5.next = l6;
        getbackNword(l1,2);
        System.out.println("原始链表元素如下：");
        while(l1!= null){
            System.out.print(l1.val);
            l1 = l1.next;
        }
    }

    public  static void getbackNword(ListNode head,int n){
		ListNode p1=head;
		ListNode p2=head;
		for(int i=0;i<n-1;i++){
			p2=p2.next;
		}
        while(p2.next!=null){
            p1=p1.next;
            p2=p2.next;
        }
        System.out.println("倒数第"+n+"个元素是"+p1.val);
	}

}