Java链表和递归

删除链表的指定元素：

public class ListNode {public int val;public ListNode next;public ListNode(int x){val=x;}//链表节点的构造函数//使用arr为参数，创建一个链表，当前的ListNode为链表头节点public ListNode(int arr[]){if(arr==null||arr.length==0)throw new IllegalArgumentException("arr can not be empty");this.val=arr[0];ListNode cur=this;for(int i=1;i");cur=cur.next;}res.append("NULL");return res.toString();}
}

　　第一种方法：

public class Solution {public ListNode removeElements(ListNode head,int val){while(head!=null&& head.val==val){
//		   ListNode delNode=head;
//		   head=head.next;
//		   delNode.next=null;head=head.next;}if(head==null)return null;ListNode prev=head;while(prev.next!=null){if(prev.next.val==val){
//	    		ListNode delNode=prev.next;
//	    		prev.next=delNode.next;
//	    		delNode.next=null;prev.next=prev.next.next; }else{prev=prev.next;}}return head;}public static void main(String[] args){int[] nums={1,2,3,4,5,6};ListNode head=new ListNode(nums);System.out.println(head);ListNode res=(new Solution()).removeElements(head, 6);System.out.println(res);}
}

　　使用头节点：

public class Solution2 {public ListNode removeElements(ListNode head,int val){ListNode dummyHead=new ListNode(-1);dummyHead.next=head;ListNode prev=dummyHead;while (prev.next!=null) {if(prev.next.val==val)prev.next=prev.next.next;elseprev=prev.next;}return head;}public static void main(String[] args){int[] nums={1,2,3,4,5,6};ListNode head=new ListNode(nums);System.out.println(head);ListNode res=(new Solution2()).removeElements(head, 6);System.out.println(res);}
}

　　实现求数组递归的算法：

public class Sum {public static int sum(int[] arr){return sum(arr,0);}//计算arr[l...n]这个区间内所有数字的和private static int sum(int[] arr,int l){if(l==arr.length)return 0;return arr[l]+sum(arr,l+1);}public static void main(String[] args){int[] nums={1,2,3,4,5,6,7,8};System.out.println(sum(nums));}
}

　　用递归实现删除链表中的元素：

public class Solution3 {public ListNode removeElements(ListNode head,int val){if(head==null)return null;head.next = removeElements(head.next, val);return head.val==val? head.next:head;}public static void main(String[] args){int[] nums={1,2,3,4,5,6};ListNode head=new ListNode(nums);System.out.println(head);ListNode res=(new Solution3 ()).removeElements(head, 6);System.out.println(res);}
}　

打印执行过程：

public class Solution3 {
public ListNode removeElements(ListNode head,int val,int depth){
String depthString=generateDepthString(depth);
System.out.println(depthString);
System.out.println("Call:remove "+val+"in "+head);if(head==null){
System.out.print(depthString);
System.out.println("Call:remove "+val+"in "+head);
return null;
}ListNode res=removeElements(head.next, val,depth+1);
System.out.print(depthString);
System.out.println("After remove "+val+":"+res);
ListNode ret;
if(head.val==val)
ret=res;
else{
head.next=res;
ret=head;
}
System.out.print(depthString);
System.out.println("Return:"+ret);
return ret;
}private String generateDepthString(int depth){
StringBuilder res=new StringBuilder();
for(int i=0;i