@Override
    public String toString() {
        return "SparseSketch{" +
                "values=" + values +
                '}';
    }
Serializes the Sketch.

Serialization format
==================
Number of elements:      4 bytes
Elements:            N * 4 bytes
@param buf Serializer
searchlib/src/main/java/com/yahoo/searchlib/aggregation/hll/SparseSketch.java