【剑指Offer】14、链表中倒数第k个结点

 1 public static ListNode FindKthToTail(ListNode head,int k) {
 2         if(head==null||k<=0){
 3             return null;
 4         }
 5         Stack<ListNode> stack = new Stack<>();
 6         while (head!=null){
 7             stack.push(head);
 8             head=head.next;
 9         }
10         if(k>stack.size()){
11             return null;
12         }
13         while (!((--k) ==0)){
14             stack.pop();
15         }
16         ListNode backTh=stack.pop();
17         return backTh;
18     }

 1 //首先定义两个指向链表头的指针p  ,q;先令p指向正数第k节点，然后两个指针同时向后移动，
 2      //直到p为最后一个，q此时移动了总长度-k个节点，即q指向倒数第k个节点。当k为零或节点为空返回。
 3     public static ListNode FindKthToTail01(ListNode head,int k) {
 4         ListNode p, q;
 5         p = q = head;
 6         int i = 0;
 7         for (; p != null; i++) {
 8             if (i >= k) {
 9                 q = q.next;
10             }
11             p = p.next;
12         }
13         return i < k ? null : q;
14     }

 1 public static class ListNode{
 2         int val=0;
 3         ListNode next=null;
 4         public ListNode(int val){
 5             this.val=val;
 6         }
 7     }
 8  public static ListNode createList(int[] list){
 9         ListNode head = new ListNode(-1);
10         ListNode current=head;
11         for(int i=0;i<list.length;i++){
12             current.next=new ListNode(list[i]);
13             current=current.next;
14         }
15         return head.next;
16     }

1 public static void main(String[] args) {
2         int[] list={1,2,3,4,5,6};
3         int k=4;
4         ListNode head= createList(list);
5         ListNode kth = FindKthToTail02(head, k);
6         System.out.println(kth.val);
7     }
8 输出: 3