LeetCode24-Swap Nodes in Pairs

Description

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.
Note:

Your algorithm should use only constant extra space.
You may not modify the values in the list's nodes, only nodes itself may be changed.

Mind Path

Note1: Using only constant extra space means that we can't use recusive solutions.
Note2: Not modifying the values means that we can only swap nodes to do the swap.

Solution

package com.dylan.leetcode;

import java.util.List;

import org.junit.Assert;
import org.junit.Test;

/**
* Created by liufengquan on 2018/8/11.
*/
public class SwapNodesInPairs {
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode newHead = new ListNode(0);
ListNode temp = newHead;
temp.next = head;
while (head != null && head.next != null) {
temp.next = head.next;
ListNode next = head.next.next;
temp.next.next = head;
head = next;
temp = temp.next.next;
temp.next = head;
}
return newHead.next;
}

@Test
public void testSwapSwapPairs() {
ListNode listNode = init(1);
Assert.assertEquals(listNode, swapPairs(listNode));

ListNode second = init(2);

ListNode swapped = swapPairs(second);
Assert.assertEquals(1, swapped.val);
Assert.assertEquals(0, swapped.next.val);

ListNode third = init(3);
ListNode thirdSwap = swapPairs(third);
Assert.assertEquals(1, thirdSwap.val);
Assert.assertEquals(0, thirdSwap.next.val);
Assert.assertEquals(2, thirdSwap.next.next.val);
}

public ListNode init(int n) {
ListNode head = new ListNode(1);

ListNode temp = head;
for (int i = 0; i < n; i++) {
temp.next = new ListNode(i);
temp = temp.next;
}

return head.next;
}
}