[Leetcode] Swap Nodes in Pairs (Java)

Given a linked list, swap every two adjacent nodes and return its head.

For example,

Given

1->2->3->4

, you should return the list as

2->1->4->3

.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

头脑发热，保存了好多的中间ListNode变量。。

public class SwapNodesinPairs {
static class ListNode {
int val;
ListNode next;
ListNode(int x) {
val = x;
next = null;
}
}
public ListNode swapPairs(ListNode head) {
if(head==null)
return null;
ListNode index = head;
int k=0;
while(index!=null){
k++;
index=index.next;
}
index = head;
ListNode res = new ListNode(-1);
res.next = head;
if(k%2==0){
ListNode pre = null;
ListNode post = null;
ListNode temp = res;
while(index!=null){
pre = index;
post = index.next;
index = index.next.next;
temp.next = post;
post.next = pre;
pre.next = index;
temp = pre;
}
}else {
ListNode pre = null;
ListNode post = null;
ListNode temp =res;
while(index.next!=null){
pre = index;
post = index.next;
index = index.next.next;
if(index!=null) {
temp.next= post;
post.next = pre;
pre.next = index;
temp = pre;
}
}
}
return res.next;
}
public static void main(String[] args) {
ListNode head = new ListNode(1);
ListNode l2 = new ListNode(2);
head.next = l2;
head = new SwapNodesinPairs().swapPairs(head);
System.out.print(head.val);
head=head.next;
while(head!=null) {
System.out.print("->"+head.val);
head=head.next;
}
}
}