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// https://leetcode.com/problems/swap-nodes-in-pairs

// T: O(n)

// S: O(1)

public class SwapNodesInPairs {

private static class ListNode {

int val;

ListNode next;

ListNode(int val, ListNode next) {

this.val = val;

this.next = next;

}

public ListNode swapPairs(ListNode head) {

if (head == null || head.next == null) return head;

ListNode newHead = head.next, a = head, b = head.next.next, temp;

head.next.next = head;

while (a != null) {

a.next = (b == null || b.next == null ? b : b.next);

temp = (b == null || b.next == null ? null : b.next.next);

if (b != null && b.next != null) b.next.next = b;

a = b;

b = temp;

}

return newHead;

}