diff --git a/README.md b/README.md
index 965e49c..f38e05b 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# 99+ Coding Interview Problems with Solutions
+# 100+ Coding Interview Problems with Solutions
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/hoanhan101/algo)
 ](https://goreportcard.com/report/github.com/hoanhan101/algo)
@@ -187,6 +187,7 @@ that you can prepare better for your next coding interviews.
     - [Reverse a linked list](gtci/reverse_list_test.go)
   - Tree breath first search
     - [Binary tree level order traversal](gtci/level_order_traversal_test.go)
+    - [Zigzag traversal](gtci/zigzag_traversal_test.go)
 - **Other**
   - Data structures
     - Linked List
diff --git a/gtci/zigzag_traversal_test.go b/gtci/zigzag_traversal_test.go
new file mode 100644
index 0000000..7996819
--- /dev/null
+++ b/gtci/zigzag_traversal_test.go
@@ -0,0 +1,115 @@
+/*
+Problem:
+- Given a binary tree, populate the values of all nodes of each level
+  in a zigzag order in separate sub-arrays.
+
+Example:
+- Input:
+      1
+	2   3
+  4       5
+  Output: [][]interface{}{[]interface{}{1}, []interface{}{3, 2}, []interface{}{4, 5}}
+
+Approach:
+- Start by pushing the root node to a queue.
+- Keep iterating until the queue is empty.
+- At each step,
+  - use a linked list to push front or back depending on the zigzag direction
+  - enqueue its left and right child
+
+Cost:
+- O(n) time, O(n) space.
+*/
+
+package gtci
+
+import (
+	"testing"
+
+	"github.com/hoanhan101/algo/common"
+)
+
+func TestZigzagTraverse(t *testing.T) {
+	t1 := &common.TreeNode{}
+
+	t2 := &common.TreeNode{nil, 1, nil}
+
+	t3 := &common.TreeNode{nil, 1, nil}
+	t3.Left = &common.TreeNode{nil, 2, nil}
+	t3.Left.Right = &common.TreeNode{nil, 3, nil}
+
+	t4 := &common.TreeNode{nil, 1, nil}
+	t4.Left = &common.TreeNode{nil, 2, nil}
+	t4.Right = &common.TreeNode{nil, 3, nil}
+
+	t5 := &common.TreeNode{nil, 1, nil}
+	t5.Left = &common.TreeNode{nil, 2, nil}
+	t5.Right = &common.TreeNode{nil, 3, nil}
+	t5.Left.Left = &common.TreeNode{nil, 4, nil}
+	t5.Right.Right = &common.TreeNode{nil, 5, nil}
+
+	tests := []struct {
+		in       *common.TreeNode
+		expected [][]interface{}
+	}{
+		{t1, [][]interface{}{[]interface{}{0}}},
+		{t2, [][]interface{}{[]interface{}{1}}},
+		{t3, [][]interface{}{[]interface{}{1}, []interface{}{2}, []interface{}{3}}},
+		{t4, [][]interface{}{[]interface{}{1}, []interface{}{3, 2}}},
+		{t5, [][]interface{}{[]interface{}{1}, []interface{}{3, 2}, []interface{}{4, 5}}},
+	}
+
+	for _, tt := range tests {
+		common.Equal(
+			t,
+			tt.expected,
+			zigzagTraverse(tt.in),
+		)
+	}
+}
+
+func zigzagTraverse(root *common.TreeNode) [][]interface{} {
+	out := [][]interface{}{}
+
+	if root == nil {
+		return out
+	}
+
+	// initialize a linked list with the root.
+	queue := common.NewQueue()
+	queue.Push(root)
+	leftToRight := true
+
+	for queue.Size() > 0 {
+		levelSize := queue.Size()
+		currentLevel := common.NewList()
+
+		for i := 0; i < levelSize; i++ {
+			// pop the queue and cache that value to its current level.
+			current := queue.Pop().(*common.TreeNode)
+
+			if leftToRight {
+				currentLevel.PushBack(current.Value)
+			} else {
+				currentLevel.PushFront(current.Value)
+			}
+
+			// push its left child.
+			if current.Left != nil {
+				queue.Push(current.Left)
+			}
+
+			// push its right child.
+			if current.Right != nil {
+				queue.Push(current.Right)
+			}
+		}
+
+		out = append(out, currentLevel.Slice())
+
+		// reverse its direction.
+		leftToRight = !leftToRight
+	}
+
+	return out
+}