BM7 链表中环的入口结点 ​

/*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)
 *     }
 * }
 */
/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 * 
 * @param pHead ListNode类 
 * @return ListNode类
 */
export function EntryNodeOfLoop(pHead: ListNode): ListNode {
    // write code here
    var fast: ListNode = pHead;
    var slow: ListNode = pHead;
    
    while(fast && fast.next) {
        fast = fast.next.next;
        slow = slow.next;
        
//         可以相遇
        if(fast ===slow) {
//             重置fast 到 pHead
            fast = pHead;
            
            while(fast !== slow) {
                fast = fast.next;
                slow = slow.next;
            }
            return fast;
        }
    }
    
    return null;
    
}