题目描述
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:
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.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
xplanation: There is no cycle in the linked list.
解题思路1: 暴力破解
这个题目是判断链表是否存在环. 第一种方法: 我们可以使用哈希表,将遍历过的item存起来, 当我们在哈希表中再次拿到这个item的时候, 就说明已经有环了
空间复杂度: O(n)
示例代码1:
func hasCycle(_ head: ListNode?) -> Bool {
guard let _ = head else {
return false
}
var hash: [ListNode: Int] = [:]
var h = head
while h != nil {
if hash[h!] == nil {
hash[h!] = 1
h = h?.next
}else {
return true
}
}
return false
}
解题思路2: 快慢指针
我们可以使用快慢指针来做这个题目, 设定2个指针, 快指针每次移动2格, 慢指针每次移动1格, 当2个指针重叠时, 就说明成环了
空间复杂度: O(1)
示例代码2:
func hasCycle(_ head: ListNode?) -> Bool {
guard let h = head else {
return false
}
// 定义快慢指针
var slow: ListNode? = h
var fast: ListNode? = h.next
while slow != fast {
if slow == nil || fast == nil {
return false
}else {
slow = slow?.next // 慢指针每次移动1格
fast = fast?.next?.next // 快指针每次移动2格
}
}
return true
}
