链表_链表中环的入口节点|算法专题

function EntryNodeOfLoop(pHead) {
      if (!pHead || !pHead.next) {
        return null;
      }
      let P1 = pHead.next;
      let P2 = pHead.next.next;
      // 1.判断是否有环
      while (P1 != P2) {
        if (P2 === null || P2.next === null) {
          return null;
        }
        P1 = P1.next;
        P2 = P2.next.next;
      }
      // 2.获取环的长度
      let temp = P1;
      let length = 1;
      P1 = P1.next;
      while (temp != P1) {
        P1 = P1.next;
        length++;
      }
      // 3.找公共节点
      P1 = P2 = pHead;
      while (length-- > 0) {
        P2 = P2.next;
      }
      while (P1 != P2) {
        P1 = P1.next;
        P2 = P2.next;
      }
      return P1;
    }