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Solution26.java

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/*

* Swap Nodes in Pairs

* Given 1->2->3->4, you should return the list as 2->1->4->3

* should use only constant space

* You may not modify the values in the list, only nodes itself can be changed

*/

public class Solution26 {

static class ListNode {

int val;

ListNode next;

ListNode(int x) {

val = x;

next = null;

}

}

public ListNode swapPairs(ListNode head) {

if (head != null && head.next != null) {

ListNode node = head;

ListNode result = head.next;

ListNode n = head.next.next;

result.next = node;

while (n != null && n.next != null) {

ListNode pre = n.next.next;

node.next = n.next;

node.next.next = n;

node = n;

n = pre;

}

node.next = n == null ? null : n;

return result;

}

return head;

}

public static void main(String ...args) {

Solution26 s26 = new Solution26();

ListNode head = new ListNode(1);

head.next = new ListNode(2);

head.next.next = new ListNode(3);

head.next.next.next = new ListNode(4);

head.next.next.next.next = new ListNode(5);

head = s26.swapPairs(head);

while (head != null) {

System.out.println(head.val);

head = head.next;

}

}

}