142.环形链表-ii

/*
 * @lc app=leetcode.cn id=142 lang=typescript
 *
 * [142] 环形链表 II
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function detectCycle(head: ListNode | null): ListNode | null {
    if (head == null) return null;
	let map: Map<ListNode, string> = new Map();

	let current: ListNode | null = head;
	while (current.next) {
		if (map.has(current)) {
			// 存在
			return current;
		}
		map.set(current, "1");
		current = current.next;
	}

    return null
};
// @lc code=end