public long getLastNodeTTL(String[] nodes) {
    CacheNode node = ttlCacheTree;
    for (int i = 1; i < nodes.length; i++) {
      node = node.getChild(nodes[i]);
      if (node == null) {
        return NULL_TTL;
      }
    }
    return node.ttl;
  }
Return the ttl of path. If the path does not exist, it means that the TTL is not set, and
return NULL_TTL.
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/schema/ttl/TTLCache.java