LeetCode

Topic

• Linked List
• Recursion

Description

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

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

Example 1:

Input: head = [1,2,3,4]
Output: [2,1,4,3]

Example 2:

Input: head = []
Output: []

Example 3:

Input: head = [1]
Output: [1]

Constraints:

• The number of nodes in the list is in the range [0, 100].
• 0 <= Node.val <= 100

Follow up: Can you solve the problem without modifying the values in the list's nodes? (i.e., Only nodes themselves may be changed.)

Analysis

方法一：迭代，两节点互换值val。

方法二：递归，连接点互换。

Submission

import com.lun.util.SinglyLinkedList.ListNode;

public class SwapNodesInPairs {

	// 方法一：
	public ListNode swapPairs1(ListNode head) {
		ListNode p1 = head;

		while (p1 != null) {
			ListNode p2 = p1.next;

			if (p2 != null) {
				int temp = p1.val;
				p1.val = p2.val;
				p2.val = temp;
				p1 = p1.next;
			}
			p1 = p1.next;
		}

		return head;
	}

	// 方法二：
	public ListNode swapPairs2(ListNode node) {
		if (node == null || node.next == null)
			return node;

		ListNode next = node.next;
		node.next = swapPairs2(next.next);
		next.next = node;
		return next;
	}

}

Test

import static org.junit.Assert.*;
import static com.lun.util.SinglyLinkedList.*;
import org.junit.Test;


public class SwapNodesInPairsTest {

	@Test
	public void test() {
		SwapNodesInPairs obj = new SwapNodesInPairs();
		
		assertTrue(areTwoListEqual(obj.swapPairs1(ints2List(1, 2, 3, 4)), ints2List(2, 1, 4, 3)));
		assertTrue(areTwoListEqual(obj.swapPairs1(ints2List(1, 2, 3, 4, 5)), ints2List(2, 1, 4, 3, 5)));
		assertNull(obj.swapPairs1(null));
		assertTrue(areTwoListEqual(obj.swapPairs1(ints2List(1)), ints2List(1)));
		
		assertTrue(areTwoListEqual(obj.swapPairs2(ints2List(1, 2, 3, 4)), ints2List(2, 1, 4, 3)));
		assertTrue(areTwoListEqual(obj.swapPairs2(ints2List(1, 2, 3, 4, 5)), ints2List(2, 1, 4, 3, 5)));
		assertNull(obj.swapPairs2(null));
		assertTrue(areTwoListEqual(obj.swapPairs2(ints2List(1)), ints2List(1)));
	}
}

来源：oschina

链接：https://my.oschina.net/jallenkwong/blog/4898643