diff --git a/.DS_Store b/.DS_Store
index ae3f0d8..1fb5ebe 100644
Binary files a/.DS_Store and b/.DS_Store differ
diff --git a/longest_substring_without_repeating_char.go b/longest_substring_without_repeating_char.go
new file mode 100644
index 0000000..cec0a74
--- /dev/null
+++ b/longest_substring_without_repeating_char.go
@@ -0,0 +1,53 @@
+package gopherriz
+
+import "strings"
+
+// Given a string s, find the length of the longest substring
+// without duplicate characters.
+
+// Our solution
+
+func lengthOfLongestSubstring(s string) int {
+	max := 0
+	unique := ""
+	var currentLetter = ""
+	for _, letter := range s {
+		currentLetter = string(letter)
+		if !strings.Contains(unique, currentLetter) {
+			unique = unique + currentLetter
+			subSLen := len(unique)
+			if max < subSLen {
+				max = subSLen
+			}
+		} else {
+
+			unique = unique[strings.Index(unique, currentLetter)+1:] + currentLetter
+		}
+	}
+
+	return max
+}
+
+// another way, more efficient way to handle the same problem.
+// using constant space, with same time complexity, but much less actual code complexity
+
+func lengthOfLongestSubstringOne(s string) int {
+	Map := make(map[byte]bool)
+	Left, Max := 0, 0
+
+	for Right := 0; Right < len(s); Right++ {
+		for Map[s[Right]] {
+			delete(Map, s[Left])
+			Left++
+		}
+
+		Map[s[Right]] = true
+
+		temp := Right - Left + 1
+		if temp > Max {
+			Max = temp
+		}
+	}
+
+	return Max
+}