code: CpcSketch copy() {
      final CpcSketch copy = new CpcSketch(lgK, seed);
      copy.numCoupons = numCoupons;
      copy.mergeFlag = mergeFlag;
      copy.fiCol = fiCol;
  
      copy.windowOffset = windowOffset;
      copy.slidingWindow = (slidingWindow == null) ? null : slidingWindow.clone();
      copy.pairTable = (pairTable == null) ? null : pairTable.copy();
  
      copy.kxp = kxp;
      copy.hipEstAccum = hipEstAccum;
      return copy;
    }
docs: Returns a copy of this sketch
  @return a copy of this sketch