判定链表中是否含有环

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    if(head===null || head.next===null){
        return false;
    }
    let slow = head,fast = head;
    while(fast!=null){
        slow = slow?.next;
        fast = fast?.next?.next;
        if(slow === fast){
            return true;
        }
    }
    return false;
};

环形链表位置

算法思路：

var detectCycle = function(head) {
    if (head === null) {
        return null;
    }
    let slow = head, fast = head;
    while (fast !== null) {
        slow = slow.next;
        if (fast.next !== null) {
            fast = fast.next.next;
        } else {
            return null;
        }
        if (fast === slow) {
            let ptr = head;
            while (ptr !== slow) {
                ptr = ptr.next;
                slow = slow.next;
            }
            return ptr;
        }
    }
    return null;
};

寻找链表的倒数第 n 个元素

var removeNthFromEnd = function(head, n) {
    let fast, slow;
    fast = slow = head;
    while(n !== 0){
        fast = fast?.next;
        n--;
    }
    if(fast === null){
        return head.next;
    }
    while(fast!==null && fast.next !== null){
        fast = fast?.next;
        slow = slow?.next;
    }
    slow.next = slow.next.next;
    return head;
};