Swap Nodes in Pairs

Swap Nodes in Pairs

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.

Solution In-Place

• Use a dummyhead to avoid dealin with null cases, as usual
• Keep track of the first node in next pair
• Termination conditions: take care of odd and even node number cases

public static ListNode swapPairs(ListNode head) {
	ListNode dummyhead = new ListNode(0);
	dummyhead.next = head; //use dummyhead to deal with null case
	ListNode first = dummyhead;
	ListNode second =head;

	while(second!=null && second.next!=null){//when second is null (odd), or second next is null(even)
	ListNode tmp = second.next.next; //record next start
	second.next.next = second; //reverse
	first.next = second.next; // link to second
	second.next = tmp; // link to next start
	first=second; // move to next
	second = second.next; //move to next
	}
	return dummyhead.next;
	}