diff --git a/python_solutions/0003_longest_substring_without_repeating_character b/python_solutions/0003_longest_substring_without_repeating_character
new file mode 100644
index 0000000..b644739
--- /dev/null
+++ b/python_solutions/0003_longest_substring_without_repeating_character
@@ -0,0 +1,24 @@
+//using sliding window
+
+import java.util.HashSet;
+import java.util.Set;
+
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        // HashSet to store the current window of unique characters
+        Set<Character> set = new HashSet<>();
+        int n = s.length();
+        int ans = 0, i = 0, j = 0;
+        while (i < n && j < n) {
+            // try to extend the range [i, j]
+            if (!set.contains(s.charAt(j))){
+                set.add(s.charAt(j++));
+                ans = Math.max(ans, j - i);
+            }
+            else {
+                set.remove(s.charAt(i++));
+            }
+        }
+        return ans;
+    }
+}