Procedures are good or bad:
The first principle is: Speed up
The second principle is: less memory
The third principle is: data should be small
There are several ways of timing programs
1) clock() function of C++, header file is time.h
This timing method is based on the CPU clock as the timing unit, also known as wall time.
The timing method is:
clock_t start=clock()
clock_t finish=clock()
total_time=(double)(finish-start)/CLOCKS_PER_SEC
2) Millisecond-level accuracy of <boost/timer.hpp> (best use)
timer t // declare timer, start timer
...
t.elapsed()// the elapsed time of output, in s
3) Functions of Libraries
Procedure consumed memory:
It can be obtained in the following way:
2.1)ps aux
2.2) top (best use)
2.3)pmap -d $pid
pid is the pid of the current program, which can be obtained by 2.1)

The first way of thinking
Using virtual header nodes

package Soution;

public class Soution1 {
	public ListNode removeElements(ListNode head,int val) {
		while(head!=null&& head.val==val) {
			ListNode delNode = head;
			head = head.next;
			delNode.next =null;
		}
		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;
			}
			else
				prev = prev.next;
		}
		return head;
	}
}

The second way of thinking
Do not use virtual header nodes

package Soution;

public class Soution11 {
	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;
				
			}
			else
				prev = prev.next;
		}
		return dummyHead.next;
	}
	public static void main(String[] args) {
		int[] nums = {1,2,6,3,4,5,6};
		ListNode head = new ListNode(nums);
		System.out.println(head);
		
		ListNode res = (new Soution11().removeElements(head,6));
		System.out.println(res);
		
		
	}
}

The third way of thinking
recursion

package Soution;

public class Soution111 {
	public ListNode removeElements(ListNode head,int val) {
	if(head==null) {
		return null;
	}
	ListNode res =removeElements(head.next,val);
	if(head.val==val)
		return res;
	else {
		head.next =res;
		return head;
	}
	
	}
	public static void main(String[] args) {
		int[] nums = {1,2,6,3,4,5,6};
		ListNode head = new ListNode(nums);
		System.out.println(head);
		
		ListNode res = (new Soution111().removeElements(head,6));
		System.out.println(res);
		
	}

}

Code optimization

package Soution;

public class YouHuaSoution111 {
	public ListNode removeElements(ListNode head,int val) {
		if(head==null) {
			return null;
		}
		head.next =removeElements(head.next,val);
		if(head.val==val)
			return head.next;
		else {
			head.next =head;
			return head;
		}
		
		}
		public static void main(String[] args) {
			int[] nums = {1,2,6,3,4,5,6};
			ListNode head = new ListNode(nums);
			System.out.println(head);
			
			ListNode res = (new Soution111().removeElements(head,6));
			System.out.println(res);
			
		}

	}

Continue to optimize

package Soution;

public class YouHuaSoution111 {
	public ListNode removeElements(ListNode head,int val) {
		if(head==null) {
			return null;
		}
		head.next =removeElements(head.next,val);
//		if(head.val==val)
//			return head.next;
//		else {
//			head.next =head;
//			return head;
//		}
		return head.val ==val?head.next:head;
		
		}
		public static void main(String[] args) {
			int[] nums = {1,2,6,3,4,5,6};
			ListNode head = new ListNode(nums);
			System.out.println(head);
			
			ListNode res = (new Soution111().removeElements(head,6));
			System.out.println(res);
			
		}

	}

Like me can pay attention to me, we can exchange learning together.

Public address:

Let me fall in love with it Computer

qq group: 473989408