diff --git a/addbatch.go b/addbatch.go
new file mode 100644
index 0000000..49a4426
--- /dev/null
+++ b/addbatch.go
@@ -0,0 +1,256 @@
+package arbo
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+)
+
+/*
+
+
+AddBatch design
+===============
+
+
+CASE A: Empty Tree --> if tree is empty (root==0)
+=================================================
+- Build the full tree from bottom to top (from all the leaf to the root)
+
+
+CASE B: ALMOST CASE A, Almost empty Tree --> if Tree has numLeafs < numBuckets
+==============================================================================
+- Get the Leafs (key & value) (iterate the tree from the current root getting
+the leafs)
+- Create a new empty Tree
+- Do CASE A for the new Tree, giving the already existing key&values (leafs)
+from the original Tree + the new key&values to be added from the AddBatch call
+
+      R
+     / \
+    A   *
+       / \
+      B   C
+
+
+CASE C: ALMOST CASE B --> if Tree has few Leafs (but numLeafs>=numBuckets)
+==============================================================================
+- Use A, B, G, F as Roots of subtrees
+- Do CASE B for each subtree
+- Then go from L to the Root
+
+              R
+             /  \
+            /    \
+           /      \
+          *        *
+         / |      / \
+        /  |     /   \
+       /   |    /     \
+L:    A    B   G       D
+              / \
+             /   \
+            /     \
+           C      *
+                 / \
+                /   \
+               /     \
+              D      E
+
+
+
+CASE D: Already populated Tree
+==============================
+- Use A, B, C, D as subtree
+- Sort the Keys in Buckets that share the initial part of the path
+- For each subtree add there the new leafs
+
+              R
+             /  \
+            /    \
+           /      \
+          *        *
+         / |      / \
+        /  |     /   \
+       /   |    /     \
+L:    A    B   C       D
+     /\   /\  / \     / \
+    ...  ... ... ... ... ...
+
+
+CASE E: Already populated Tree Unbalanced
+=========================================
+- Need to fill M1 and M2, and then will be able to use CASE D
+	- Search for M1 & M2 in the inputed Keys
+	- Add M1 & M2 to the Tree
+	- From here can use CASE D
+
+              R
+             /  \
+            /    \
+           /      \
+          *        *
+           |        \
+           |         \
+           |          \
+L:    M1   *   M2      *        (where M1 and M2 are empty)
+          / |         /
+         /  |        /
+        /   |       /
+       A    *      *
+           / \     | \
+          /   \    |  \
+         /     \   |   \
+        B      *   *   C
+              / \  |\
+           ... ... | \
+                   |  \
+                   D  E
+
+
+
+Algorithm decision
+==================
+- if nLeafs==0 (root==0): CASE A
+- if nLeafs<nBuckets: CASE B
+- if nLeafs>=nBuckets && nLeafs < minLeafsThreshold: CASE C
+- else: CASE D & CASE E
+
+
+- Multiple tree.Add calls: O(n log n)
+	- Used in: cases A, B, C
+- Tree from bottom to top: O(log n)
+	- Used in: cases D, E
+
+*/
+
+// AddBatchOpt is the WIP implementation of the AddBatch method in a more
+// optimized approach.
+func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
+	t.updateAccessTime()
+	t.Lock()
+	defer t.Unlock()
+
+	// TODO if len(keys) is not a power of 2, add padding of empty
+	// keys&values. Maybe when len(keyvalues) is not a power of 2, cut at
+	// the biggest power of 2 under the len(keys), add those 2**n key-values
+	// using the AddBatch approach, and then add the remaining key-values
+	// using tree.Add.
+
+	kvs, err := t.keysValuesToKvs(keys, values)
+	if err != nil {
+		return nil, err
+	}
+
+	t.tx, err = t.db.NewTx()
+	if err != nil {
+		return nil, err
+	}
+
+	// if nLeafs==0 (root==0): CASE A
+	e := make([]byte, t.hashFunction.Len())
+	if bytes.Equal(t.root, e) {
+		// CASE A
+		// sort keys & values by path
+		sortKvs(kvs)
+		return t.buildTreeBottomUp(kvs)
+	}
+
+	return nil, fmt.Errorf("UNIMPLEMENTED")
+}
+
+type kv struct {
+	pos     int // original position in the array
+	keyPath []byte
+	k       []byte
+	v       []byte
+}
+
+// compareBytes compares byte slices where the bytes are compared from left to
+// right and each byte is compared by bit from right to left
+func compareBytes(a, b []byte) bool {
+	// WIP
+	for i := 0; i < len(a); i++ {
+		for j := 0; j < 8; j++ {
+			aBit := a[i] & (1 << j)
+			bBit := b[i] & (1 << j)
+			if aBit > bBit {
+				return false
+			} else if aBit < bBit {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// sortKvs sorts the kv by path
+func sortKvs(kvs []kv) {
+	sort.Slice(kvs, func(i, j int) bool {
+		return compareBytes(kvs[i].keyPath, kvs[j].keyPath)
+	})
+}
+
+func (t *Tree) keysValuesToKvs(ks, vs [][]byte) ([]kv, error) {
+	if len(ks) != len(vs) {
+		return nil, fmt.Errorf("len(keys)!=len(values) (%d!=%d)",
+			len(ks), len(vs))
+	}
+	kvs := make([]kv, len(ks))
+	for i := 0; i < len(ks); i++ {
+		keyPath := make([]byte, t.hashFunction.Len())
+		copy(keyPath[:], ks[i])
+		kvs[i].pos = i
+		kvs[i].keyPath = ks[i]
+		kvs[i].k = ks[i]
+		kvs[i].v = vs[i]
+	}
+
+	return kvs, nil
+}
+
+// keys & values must be sorted by path, and must be length multiple of 2
+// TODO return index of failed keyvaules
+func (t *Tree) buildTreeBottomUp(kvs []kv) ([]int, error) {
+	// build the leafs
+	leafKeys := make([][]byte, len(kvs))
+	for i := 0; i < len(kvs); i++ {
+		// TODO handle the case where Key&Value == 0
+		leafKey, leafValue, err := newLeafValue(t.hashFunction, kvs[i].k, kvs[i].v)
+		if err != nil {
+			return nil, err
+		}
+		// store leafKey & leafValue to db
+		if err := t.tx.Put(leafKey, leafValue); err != nil {
+			return nil, err
+		}
+		leafKeys[i] = leafKey
+	}
+	r, err := t.upFromKeys(leafKeys)
+	if err != nil {
+		return nil, err
+	}
+	t.root = r
+	return nil, nil
+}
+
+func (t *Tree) upFromKeys(ks [][]byte) ([]byte, error) {
+	if len(ks) == 1 {
+		return ks[0], nil
+	}
+
+	var rKs [][]byte
+	for i := 0; i < len(ks); i += 2 {
+		// TODO handle the case where Key&Value == 0
+		k, v, err := newIntermediate(t.hashFunction, ks[i], ks[i+1])
+		if err != nil {
+			return nil, err
+		}
+		// store k-v to db
+		if err = t.tx.Put(k, v); err != nil {
+			return nil, err
+		}
+		rKs = append(rKs, k)
+	}
+	return t.upFromKeys(rKs)
+}
diff --git a/addbatch_test.go b/addbatch_test.go
new file mode 100644
index 0000000..64dfb4d
--- /dev/null
+++ b/addbatch_test.go
@@ -0,0 +1,51 @@
+package arbo
+
+import (
+	"fmt"
+	"math/big"
+	"testing"
+	"time"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/iden3/go-merkletree/db/memory"
+)
+
+func TestAddBatchCaseA(t *testing.T) {
+	c := qt.New(t)
+
+	nLeafs := 1024
+
+	tree, err := NewTree(memory.NewMemoryStorage(), 100, HashFunctionPoseidon)
+	c.Assert(err, qt.IsNil)
+	defer tree.db.Close()
+
+	start := time.Now()
+	for i := 0; i < nLeafs; i++ {
+		k := BigIntToBytes(big.NewInt(int64(i)))
+		v := BigIntToBytes(big.NewInt(int64(i * 2)))
+		if err := tree.Add(k, v); err != nil {
+			t.Fatal(err)
+		}
+	}
+	fmt.Println(time.Since(start))
+
+	tree2, err := NewTree(memory.NewMemoryStorage(), 100, HashFunctionPoseidon)
+	c.Assert(err, qt.IsNil)
+	defer tree2.db.Close()
+
+	var keys, values [][]byte
+	for i := 0; i < nLeafs; i++ {
+		k := BigIntToBytes(big.NewInt(int64(i)))
+		v := BigIntToBytes(big.NewInt(int64(i * 2)))
+		keys = append(keys, k)
+		values = append(values, v)
+	}
+	start = time.Now()
+	indexes, err := tree2.AddBatchOpt(keys, values)
+	c.Assert(err, qt.IsNil)
+	fmt.Println(time.Since(start))
+	c.Check(len(indexes), qt.Equals, 0)
+
+	// check that both trees roots are equal
+	c.Check(tree2.Root(), qt.DeepEquals, tree.Root())
+}