diff --git a/top-100-liked/26/26_test.go b/top-100-liked/26/26_test.go
new file mode 100644
index 0000000..824d76b
--- /dev/null
+++ b/top-100-liked/26/26_test.go
@@ -0,0 +1,101 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/linked-list-cycle-ii/?envType=study-plan-v2&envId=top-100-liked
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+// 哈希表
+// func detectCycle(head *ListNode) *ListNode {
+// 	_map := make(map[*ListNode]bool)
+// 	for head != nil {
+// 		if _map[head] {
+// 			return head
+// 		}
+// 		_map[head] = true
+// 		head = head.Next
+// 	}
+// 	return nil
+// }
+
+// 快慢指针法
+// 核心推导如下：
+//
+// 设定变量
+//
+// • a = 头节点到入环点的距离
+// • b = 入环点到相遇点的距离
+// • c = 相遇点回到入环点的距离（环长 = b + c）
+//
+// 推导
+//
+// 相遇时：
+//
+// • slow 走了 a + b
+// • fast 走了 a + b + n(b + c)（多走了 n 整圈）
+// • 因为 fast 速度是 slow 的 2 倍：2(a + b) = a + b + n(b + c)
+//
+// 化简得：
+//
+// a + b = n(b + c)
+//
+//	a = n(b + c) - b
+//	a = (n-1)(b + c) + c
+//
+// 等式的含义
+//
+// a = (n-1)(b + c) + c 意味着：
+// 从头节点走 a 步 = 从相遇点走 (n-1) 整圈 + 再走 c 步
+// 而 c 步恰好就是从相遇点回到入环点的距离。
+//
+// 所以
+//
+// • ptr 从头出发，走 a 步到达入环点
+// • slow 从相遇点出发，走 (n-1) 圈 + c 步，也到达入环点
+// • 两者速度相同、同时出发，必然在入环点相遇
+//
+// 这就是为什么不需要知道 a、b、c 的具体值，只需要让两个指针同速走，就能找到入口。
+func detectCycle(head *ListNode) *ListNode {
+	slow, fast := head, head
+	for fast != nil {
+		slow = slow.Next
+		if fast.Next == nil {
+			return nil
+		}
+		fast = fast.Next.Next
+		if fast == slow {
+			p := head
+			for p != slow {
+				p = p.Next
+				slow = slow.Next
+			}
+			return p
+		}
+	}
+	return nil
+}
+
+func Test(t *testing.T) {
+	input := &ListNode{
+		Val: 3,
+		Next: &ListNode{
+			Val: 2,
+			Next: &ListNode{
+				Val: 0,
+				Next: &ListNode{
+					Val:  -4,
+					Next: nil,
+				},
+			},
+		},
+	}
+	input.Next.Next.Next.Next = input.Next
+	fmt.Println(detectCycle(input))
+}
diff --git a/top-100-liked/27/27_test.go b/top-100-liked/27/27_test.go
new file mode 100644
index 0000000..06de6ea
--- /dev/null
+++ b/top-100-liked/27/27_test.go
@@ -0,0 +1,63 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/merge-two-sorted-lists/description/?envType=study-plan-v2&envId=top-100-liked
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+func mergeTwoLists(list1 *ListNode, list2 *ListNode) *ListNode {
+	p1, p2 := list1, list2
+	p := &ListNode{}
+	res := p
+	for p1 != nil && p2 != nil {
+		if p1.Val <= p2.Val {
+			p.Next = p1
+			p = p.Next
+			p1 = p1.Next
+		} else {
+			p.Next = p2
+			p = p.Next
+			p2 = p2.Next
+		}
+	}
+	for p1 != nil {
+		p.Next = p1
+		p = p.Next
+		p1 = p1.Next
+	}
+	for p2 != nil {
+		p.Next = p2
+		p = p.Next
+		p2 = p2.Next
+	}
+	return res.Next
+}
+
+func Test(t *testing.T) {
+	input1 := &ListNode{
+		Val: -9,
+		Next: &ListNode{
+			Val:  3,
+			Next: nil,
+		},
+	}
+	input2 := &ListNode{
+		Val: 5,
+		Next: &ListNode{
+			Val:  7,
+			Next: nil,
+		},
+	}
+	output := mergeTwoLists(input1, input2)
+	for output != nil {
+		fmt.Println(output.Val)
+		output = output.Next
+	}
+}
diff --git a/top-100-liked/28/28_test.go b/top-100-liked/28/28_test.go
new file mode 100644
index 0000000..9ccab7c
--- /dev/null
+++ b/top-100-liked/28/28_test.go
@@ -0,0 +1,62 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/add-two-numbers/description/?envType=study-plan-v2&envId=top-100-liked
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
+	dummy := &ListNode{}
+	p := dummy
+	carry := 0
+	for l1 != nil || l2 != nil || carry > 0 {
+		sum := carry
+		if l1 != nil {
+			sum += l1.Val
+			l1 = l1.Next
+		}
+		if l2 != nil {
+			sum += l2.Val
+			l2 = l2.Next
+		}
+		p.Next = &ListNode{Val: sum % 10}
+		p = p.Next
+		carry = sum / 10
+	}
+	return dummy.Next
+}
+
+func Test(t *testing.T) {
+	input1 := &ListNode{
+		Val: 2,
+		Next: &ListNode{
+			Val: 4,
+			Next: &ListNode{
+				Val:  3,
+				Next: nil,
+			},
+		},
+	}
+	input2 := &ListNode{
+		Val: 5,
+		Next: &ListNode{
+			Val: 6,
+			Next: &ListNode{
+				Val:  4,
+				Next: nil,
+			},
+		},
+	}
+	output := addTwoNumbers(input1, input2)
+	for output != nil {
+		fmt.Println(output.Val)
+		output = output.Next
+	}
+}
diff --git a/top-100-liked/29/29_test.go b/top-100-liked/29/29_test.go
new file mode 100644
index 0000000..163df75
--- /dev/null
+++ b/top-100-liked/29/29_test.go
@@ -0,0 +1,83 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/remove-nth-node-from-end-of-list/description/?envType=study-plan-v2&envId=top-100-liked
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+// func removeNthFromEnd(head *ListNode, n int) *ListNode {
+// 	pre := &ListNode{Next: head}
+// 	p := head
+// 	l := 0
+// 	for p != nil {
+// 		l++
+// 		p = p.Next
+// 	}
+// 	p1 := pre
+// 	p2 := pre.Next.Next
+// 	for range l - n {
+// 		p1 = p1.Next
+// 		p2 = p2.Next
+// 	}
+// 	p1.Next = p2
+// 	return pre.Next
+// }
+
+// 双指针
+func removeNthFromEnd(head *ListNode, n int) *ListNode {
+	dummy := &ListNode{0, head}
+	first, second := head, dummy
+	for range n {
+		first = first.Next
+	}
+	for first != nil {
+		first = first.Next
+		second = second.Next
+	}
+	second.Next = second.Next.Next
+	return dummy.Next
+}
+
+func Test(t *testing.T) {
+	input1 := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val: 2,
+			Next: &ListNode{
+				Val: 3,
+				Next: &ListNode{
+					Val: 4,
+					Next: &ListNode{
+						Val:  5,
+						Next: nil,
+					},
+				},
+			},
+		},
+	}
+	input2 := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val:  2,
+			Next: nil,
+		},
+	}
+	output := removeNthFromEnd(input1, 2)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+	fmt.Println()
+	output = removeNthFromEnd(input2, 1)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+}
diff --git a/top-100-liked/30/30_test.go b/top-100-liked/30/30_test.go
new file mode 100644
index 0000000..1867435
--- /dev/null
+++ b/top-100-liked/30/30_test.go
@@ -0,0 +1,89 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/swap-nodes-in-pairs/description/?envType=study-plan-v2&envId=top-100-liked
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+func swapPairs(head *ListNode) *ListNode {
+	if head == nil {
+		return nil
+	}
+	if head.Next == nil {
+		return head
+	}
+	dummy := &ListNode{0, head}
+	p1, p2, p3 := dummy, dummy.Next, dummy.Next.Next
+	for {
+		p1.Next = p3
+		p2.Next = p3.Next
+		p3.Next = p2
+		p2, p3 = p3, p2
+		for range 2 {
+			if p3.Next == nil {
+				return dummy.Next
+			}
+			p1 = p1.Next
+			p2 = p2.Next
+			p3 = p3.Next
+		}
+	}
+}
+
+// func swapPairs(head *ListNode) *ListNode {
+//     dummyHead := &ListNode{0, head}
+//     temp := dummyHead
+//     for temp.Next != nil && temp.Next.Next != nil {
+//         node1 := temp.Next
+//         node2 := temp.Next.Next
+//         temp.Next = node2
+//         node1.Next = node2.Next
+//         node2.Next = node1
+//         temp = node1
+//     }
+//     return dummyHead.Next
+// }
+
+func Test(t *testing.T) {
+	input1 := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val: 2,
+			Next: &ListNode{
+				Val: 3,
+				Next: &ListNode{
+					Val:  4,
+					Next: nil,
+				},
+			},
+		},
+	}
+	input2 := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val: 2,
+			Next: &ListNode{
+				Val:  3,
+				Next: nil,
+			},
+		},
+	}
+	output := swapPairs(input1)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+	fmt.Println()
+	output = swapPairs(input2)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+}
diff --git a/top-100-liked/31/31_test.go b/top-100-liked/31/31_test.go
new file mode 100644
index 0000000..cb32549
--- /dev/null
+++ b/top-100-liked/31/31_test.go
@@ -0,0 +1,135 @@
+package top100liked
+
+import (
+	"fmt"
+	"testing"
+)
+
+// https://leetcode.cn/problems/reverse-nodes-in-k-group/description/?envType=study-plan-v2&envId=top-100-liked
+
+type ListNode struct {
+	Val  int
+	Next *ListNode
+}
+
+// 提交记录 3
+// 你就说快不快吧
+// func reverseKGroup(head *ListNode, k int) *ListNode {
+// 	if k == 1 {
+// 		return head
+// 	}
+// 	nodes := []*ListNode{}
+// 	p := head
+// 	nodes = append(nodes, &ListNode{0, head})
+// 	for p != nil {
+// 		nodes = append(nodes, p)
+// 		p = p.Next
+// 	}
+// 	tmp := nodes[0]
+// 	for i := range (len(nodes) - 1) / k {
+// 		tmp.Next = nodes[(i+1)*k]
+// 		nodes[i*k+1].Next = nodes[(i+1)*k].Next
+// 		for j := range k - 1 {
+// 			nodes[(i+1)*k-j].Next = nodes[(i+1)*k-j-1]
+// 		}
+// 		tmp = nodes[i*k+1]
+// 	}
+// 	return nodes[0].Next
+// }
+
+func reverseKGroup(head *ListNode, k int) *ListNode {
+	dummy := &ListNode{Next: head}
+	prev := dummy
+	for {
+		// 检查剩余节点是否够 k 个
+		tail := prev
+		for range k {
+			tail = tail.Next
+			if tail == nil {
+				return dummy.Next
+			}
+		}
+		next := tail.Next // 下一组的起点
+		// 翻转 [prev, tail] 这 k 个节点
+		p := prev.Next
+		var reversedHead, reversedTail *ListNode
+		reversedTail = p // 翻转后 p 变成尾部
+		var pre *ListNode
+		for range k {
+			tmp := p.Next
+			p.Next = pre
+			pre = p
+			p = tmp
+		}
+		reversedHead = pre
+		// 接回原链表
+		prev.Next = reversedHead
+		reversedTail.Next = next
+		prev = reversedTail
+	}
+}
+
+func Test1(t *testing.T) {
+	input := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val: 2,
+			Next: &ListNode{
+				Val: 3,
+				Next: &ListNode{
+					Val: 4,
+					Next: &ListNode{
+						Val:  5,
+						Next: nil,
+					},
+				},
+			},
+		},
+	}
+	output := reverseKGroup(input, 2)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+}
+
+func Test2(t *testing.T) {
+	input := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val: 2,
+			Next: &ListNode{
+				Val: 3,
+				Next: &ListNode{
+					Val: 4,
+					Next: &ListNode{
+						Val:  5,
+						Next: nil,
+					},
+				},
+			},
+		},
+	}
+	output := reverseKGroup(input, 3)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+	fmt.Println()
+}
+
+func Test3(t *testing.T) {
+	input := &ListNode{
+		Val: 1,
+		Next: &ListNode{
+			Val:  2,
+			Next: nil,
+		},
+	}
+	output := reverseKGroup(input, 2)
+	for output != nil {
+		fmt.Print(output.Val, " ")
+		output = output.Next
+	}
+	fmt.Println()
+}