[LeetCode][JavaScript]Linked List Cycle II

Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

https://leetcode.com/problems/linked-list-cycle-ii/

 

 

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快慢指针。

https://leetcode.com/discuss/396/is-there-any-better-answer-for-the-linked-list-cycle-ii

两个指针重合说明有环。

然后把慢指针指向head，双指针每次走一步，再次重合就是结果。

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function(head) {
    var slow = head, fast = head;
    while(true){
        if(fast === null || fast.next === null){
            return null;
        }
        slow = slow.next;        
        fast = fast.next.next;
        if(slow === fast){
            slow = head;
            while(true){
                if(slow === fast){
                    return slow;
                }
                slow = slow.next;
                fast = fast.next;
            }
        }
    }
};