Given head, the head of a linked list, determine if the linked list has a cycle in it.
There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.
Return true if there is a cycle in the linked list. Otherwise, return false.
Example 1:
Input: head = [3,2,0,-4], pos = 1 Output: true Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).
Example 2:
Input: head = [1,2], pos = 0 Output: true Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.
Example 3:
Input: head = [1], pos = -1 Output: false Explanation: There is no cycle in the linked list.
Constraints:
- The number of the nodes in the list is in the range
[0, 104]. -105 <= Node.val <= 105posis-1or a valid index in the linked-list.
Follow up: Can you solve it using O(1) (i.e. constant) memory?
var hasCycle = function(head) {
if (!head) return false
let fast = head
let current = head
while(fast) {
if (!fast.next) {
return false
} else {
[fast, current] = [fast.next.next, current.next]
}
if (fast === current) {
return true
}
}
return false
};判斷是否為迴圈，有一個常見的方法 Floyd判圈算法(Floyd Cycle Detection Algorithm)，又稱龜兔賽跑算法(Tortoise and Hare Algorithm)。 這個 Youtube 影片有很詳細的解脫。
看完之後解法也呼之欲出。
需要一快一慢兩個變數。快速的變數每次往前推進兩個單位，慢數的則是每次往下一個單位，當出現盡頭時則不再檢測，排除是迴圈的可能。不段往下測試結果，只要兩個變數再次交疊在同一個位置，就可以判定是迴圈。
