24. Swap Nodes in Pairs

README.md

@@ -52,6 +52,7 @@ continually updating 😃.
 * [19. Remove Nth Node From End of List](src/0019_remove_nth_node_from_end_of_list/remove_nth_node_from_end_of_list.go)   *`two pointers`*
 * [21. Merge Two Sorted Lists](./src/0021_merge_two_sorted_lists/mergeTwoLists.go)
 * [23. Merge k Sorted Lists](src/0023_merge_k_sorted_lists/mksl.go)   *`heap`*
+* [24. Swap Nodes in Pairs](src/0024_swap_nodes_in_pairs/swap_nodes_in_pairs.go)
 * [25. Reverse Nodes in k-Group](./src/0025_reverse_nodes_in_k_group/reverse_node_k_group.go)
 * [61. Rotate List](./src/0061_rotate_list/rotate_list.go)
 

src/0024_swap_nodes_in_pairs/swap_nodes_in_pairs.go

@@ -0,0 +1,35 @@
+/*
+24. Swap Nodes in Pairs
+https://leetcode.com/problems/swap-nodes-in-pairs/
+
+Given a linked list, swap every two adjacent nodes and return its head.
+*/
+// time: 2019-01-02
+
+package swapnodesinpairs
+
+// ListNode Definition for singly-linked list.
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+// time complexity: O(n/2), where n is len of linked list.
+// space complexity: O(1)
+func swapPairs(head *ListNode) *ListNode {
+	dummy := &ListNode{}
+	dummy.Next = head
+	cur := dummy
+
+	for cur.Next != nil && cur.Next.Next != nil {
+		node1 := cur.Next
+		node2 := node1.Next
+		node3 := node2.Next
+
+		node2.Next = node1
+		node1.Next = node3
+		cur.Next = node2
+		cur = node1
+	}
+	return dummy.Next
+}

src/0024_swap_nodes_in_pairs/swap_nodes_in_pairs_test.go

@@ -0,0 +1,24 @@
+package swapnodesinpairs
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestSwapPairs(t *testing.T) {
+	head := createSingleList([]int{1, 2, 3, 4})
+	expected := createSingleList([]int{2, 1, 4, 3})
+	if res := swapPairs(head); !reflect.DeepEqual(res, expected) {
+		t.Errorf("expected %v, got %v", expected, res)
+	}
+}
+
+func createSingleList(nums []int) *ListNode {
+	head := &ListNode{}
+	cur := head
+	for _, j := range nums {
+		cur.Next = &ListNode{Val: j}
+		cur = cur.Next
+	}
+	return head.Next
+}

src/README.md

@@ -16,6 +16,7 @@
 |0020|[Valid Parentheses](0020_valid_parentheses/valid_parentheses.go)|Easy|*`string;`* *`stack`*|
 |0021|[Merge Two Sorted Lists](0021_merge_two_sorted_lists/mergeTwoLists.go)|Easy|*`linked list`*|
 |0023|[23. Merge k Sorted Lists](0023_merge_k_sorted_lists/mksl.go)|Hard|*`linked list;`**`heap`*|
+|0024|[24. Swap Nodes in Pairs](0024_swap_nodes_in_pairs/swap_nodes_in_pairs.go)|Medium|*`linked list`*|
 |0025|[Reverse Nodes in k-Group](./0025_reverse_nodes_in_k_group/reverse_node_k_group.go)|Hard|*`linked list`*|
 |0026|[Remove Duplicates from Sorted Array](0026_remove_duplicates_from_sorted_array/rdfsa.go)|Easy|*`array;`*  *`double index`*|
 |0027|[Remove Element](0027_remove_element/remove_element.go)|Easy|*`array`*|