CODE 108. Swap Nodes in Pairs

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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.

public ListNode swapPairs(ListNode head) {
	// Start typing your Java solution below
	// DO NOT write main() function
	ListNode newHead = new ListNode(0);
	ListNode tmpHead = newHead;
	ListNode tmp = head;
	while (null != tmp && null != tmp.next) {
		ListNode first = tmp;
		ListNode second = tmp.next;
		tmp = second.next;
		tmpHead.next = second;
		tmpHead.next.next = first;
		tmpHead = tmpHead.next.next;
	}
	if (null != tmp) {
		tmpHead.next = tmp;
		tmpHead = tmpHead.next;
	}
	tmpHead.next = null;
	return newHead.next;
}