I would like to ask: how to add a new node before the head of a linked list: Here is my code:
// Definition for singly-linked list.
public class ListNode {
int val;
ListNode next;
ListNode() {}
ListNode(int val) { this.val = val; }
ListNode(int val, ListNode next) { this.val = val; this.next = next; }
public void addAfter(ListNode thisnode, int x) {
ListNode newNode = new ListNode(x);
if(thisnode == null) {
//add before the head
newNode.next = this;
}//wrong here
else {
ListNode currentNext = thisnode.next;
thisnode.next = newNode;
newNode.next = currentNext;
}
return;
}//done addAfter this node
}
For example, with the input list 2 100 300 800, l.addAfter(null,500); the output should be 500 2 100 300 800 but my output is still 2 100 300 800. Thank you.
Inserting before head would change the value of head. Which you can't do from inside the method.
public ListNode addAfter(ListNode thisnode, int x) {
ListNode newNode = new ListNode(x);
if(thisnode == null) {
//add before the head
newNode.next = this;
return newNode;
} else {
ListNode currentNext = thisnode.next;
thisnode.next = newNode;
newNode.next = currentNext;
}
return this;
}
And the caller would call it like l = l.addAfter(null,500);
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