Question Definition
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.
Java Solution
public ListNode swapPairs(ListNode head) {
if(head == null || head.next == null) return head;
ListNode newHead = head.next;
ListNode pre = head;
head = head.next;
while(head != null){
ListNode temp = head.next;
head.next = pre;
if(temp != null){
if(temp.next == null){
pre.next = temp;
break;
}else{
pre.next = temp.next;
pre = temp;
head = pre.next;
}
} else{
pre.next = null;
break;
}
}
return newHead;
}
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