Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
You must do this in-place without altering the nodes' values.
For example,
Given {1,2,3,4}
, reorder it to {1,4,2,3}
.
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
void reorderList(ListNode *head) {
if(head == NULL){
return;
}
int len=0, index=0;
ListNode *p1, *p2, *q1, *q2;
p1=head;
while(p1!=NULL){
len++;
p1=p1->next;
}
if(len==1||len==2){
return;
}
if(len%2==0){
index=len/2-1;
}else{
index=len/2;
}
q1=head;
for(int i=0;i<index;i++){
q1=q1->next;
}
q2=q1->next;
q1->next=NULL;
q1 = q2->next;
q2->next=NULL;
ListNode *curr;
while(q1!=NULL){
curr=q1;
q1=q1->next;
curr->next=q2;
q2=curr;
}
p2=head;
while(q2!=NULL){
p1=p2;
p2=p1->next;
q1=q2;
q2=q1->next;
p1->next=q1;
q1->next=p2;
}
}
};