141 wrong answer

Author: aQuaYi

Algorithms/0141.linked-list-cycle/1.png

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+# [141. Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/)
+
+Given a linked list, determine if it has a cycle in it.
+
+To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
+
+Example 1:
+
+```text
+Input: head = [3,2,0,-4], pos = 1
+Output: true
+Explanation: There is a cycle in the linked list, where tail connects to the second node.
+```
+
+![1](1.png)
+
+Example 2:
+
+```text
+Input: head = [1,2], pos = 0
+Output: true
+Explanation: There is a cycle in the linked list, where tail connects to the first node.
+```
+
+![2](2.png)
+
+Example 3:
+
+```text
+Input: head = [1], pos = -1
+Output: false
+Explanation: There is no cycle in the linked list.
+```
+
+![3](3.png)
+
+Follow up:
+
+- Can you solve it using O(1) (i.e. constant) memory?

Algorithms/0141.linked-list-cycle/2.png

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+package problem0141
+
+import "github.com/aQuaYi/LeetCode-in-Go/kit"
+
+// ListNode is pre-defined...
+type ListNode = kit.ListNode
+
+func hasCycle(head *ListNode) bool {
+	if head == nil {
+		return false
+	}
+	slow, fast := head, head
+	for fast != nil && fast.Next != nil && slow != fast {
+		slow, fast = slow.Next, fast.Next.Next
+	}
+	return slow == fast && head.Next != nil
+}

Algorithms/0141.linked-list-cycle/3.png

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+package problem0141
+
+import (
+	"testing"
+
+	"github.com/aQuaYi/LeetCode-in-Go/kit"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// tcs is testcase slice
+var tcs = []struct {
+	head []int
+	pos  int
+	ans  bool
+}{
+
+	{
+		[]int{3, 2, 0, -4},
+		1,
+		true,
+	},
+
+	{
+		[]int{1, 2},
+		0,
+		true,
+	},
+
+	{
+		[]int{},
+		-1,
+		false,
+	},
+
+	{
+		[]int{1},
+		-1,
+		false,
+	},
+
+	// 可以有多个 testcase
+}
+
+func Test_hasCycle(t *testing.T) {
+	ast := assert.New(t)
+
+	for _, tc := range tcs {
+		head := kit.Ints2ListWithCycle(tc.head, tc.pos)
+		ast.Equal(tc.ans, hasCycle(head), "输入:%v", tc)
+	}
+}
+
+func Benchmark_hasCycle(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range tcs {
+			head := kit.Ints2ListWithCycle(tc.head, tc.pos)
+			hasCycle(head)
+		}
+	}
+}