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)));
}
}