diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..ec78f66
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module sortedset/m
+
+go 1.15
diff --git a/sortedset.go b/sortedset.go
index 9220857..214d317 100644
--- a/sortedset.go
+++ b/sortedset.go
@@ -41,6 +41,11 @@ type SortedSet struct {
 	dict   map[string]*SortedSetNode
 }
 
+func (this *SortedSet) Print() {
+	if this.header != nil {
+		this.header.Print()
+	}
+}
 func createNode(level int, score SCORE, key string, value interface{}) *SortedSetNode {
 	node := SortedSetNode{
 		score: score,
@@ -96,6 +101,7 @@ func (this *SortedSet) insertNode(score SCORE, key string, value interface{}) *S
 	 * if the element is already inside or not. */
 	level := randomLevel()
 
+	//level := this.level + 1
 	if level > this.level { // add a new level
 		for i := this.level; i < level; i++ {
 			rank[i] = 0
diff --git a/sortedset_test.go b/sortedset_test.go
index 233292c..90a713f 100644
--- a/sortedset_test.go
+++ b/sortedset_test.go
@@ -1,7 +1,10 @@
 package sortedset
 
 import (
+	"fmt"
+	"math/rand"
 	"testing"
+	"time"
 )
 
 func checkOrder(t *testing.T, nodes []*SortedSetNode, expectedOrder []string) {
@@ -20,16 +23,23 @@ func TestCase1(t *testing.T) {
 	sortedset := New()
 
 	sortedset.AddOrUpdate("a", 89, "Kelly")
+	sortedset.Print()
 	sortedset.AddOrUpdate("b", 100, "Staley")
+	sortedset.Print()
 	sortedset.AddOrUpdate("c", 100, "Jordon")
+	sortedset.Print()
 	sortedset.AddOrUpdate("d", -321, "Park")
+	sortedset.Print()
 	sortedset.AddOrUpdate("e", 101, "Albert")
+	sortedset.Print()
 	sortedset.AddOrUpdate("f", 99, "Lyman")
+	sortedset.Print()
 	sortedset.AddOrUpdate("g", 99, "Singleton")
+	sortedset.Print()
 	sortedset.AddOrUpdate("h", 70, "Audrey")
-
+	sortedset.Print()
 	sortedset.AddOrUpdate("e", 99, "ntrnrt")
-
+	sortedset.Print()
 	sortedset.Remove("b")
 
 	node := sortedset.GetByRank(3, false)
@@ -162,3 +172,36 @@ func TestCase2(t *testing.T) {
 	nodes = sortedset.GetByScoreRange(500, -500, nil)
 	checkOrder(t, nodes, []string{"g", "f", "e", "a", "h"})
 }
+func TestCase3(t *testing.T) {
+	sortedset := New()
+	testSet := make(map[string]int64)
+	rand.Seed(0)
+	count := 100000
+	for len(testSet) < count {
+		k := rand.Int63()
+		ks := fmt.Sprintf("%d", k)
+		if _, ok := testSet[ks]; !ok {
+			testSet[ks] = k
+		}
+	}
+
+	startts := time.Now()
+
+	for k, v := range testSet {
+		sortedset.AddOrUpdate(k, SCORE(v), v)
+	}
+
+	dur := time.Since(startts)
+	t.Logf("insert cost %v ", dur/time.Duration(count))
+
+	startts = time.Now()
+	for {
+		min := sortedset.PopMin()
+		if min == nil {
+			break
+		}
+	}
+	dur = time.Since(startts)
+	t.Logf("pop cost %v ", dur/time.Duration(count))
+
+}
diff --git a/sortedsetnode.go b/sortedsetnode.go
index b060191..1ddad8c 100644
--- a/sortedsetnode.go
+++ b/sortedsetnode.go
@@ -24,6 +24,8 @@
 
 package sortedset
 
+import "fmt"
+
 type SortedSetLevel struct {
 	forward *SortedSetNode
 	span    int64
@@ -47,3 +49,17 @@ func (this *SortedSetNode) Key() string {
 func (this *SortedSetNode) Score() SCORE {
 	return this.score
 }
+func (this *SortedSetNode) Print() {
+	for i, _ := range this.level {
+		x := this.level[i].forward
+		if x == nil {
+			continue
+		}
+		fmt.Printf("No %d", i)
+		for x != nil {
+			fmt.Printf(" %d:%p:[%d]->", x.score, x, x.level[i].span)
+			x = x.level[i].forward
+		}
+		fmt.Printf("nil\n")
+	}
+}