Add longest substring (#6007)

src/main/java/com/thealgorithms/slidingwindow/LongestSubstringWithoutRepeatingCharacters.java

@@ -0,0 +1,46 @@
+package com.thealgorithms.slidingwindow;
+import java.util.HashSet;
+
+/**
+ * The Longest Substring Without Repeating Characters algorithm finds the length of
+ * the longest substring without repeating characters in a given string.
+ *
+ * <p>
+ * Worst-case performance O(n)
+ * Best-case performance O(n)
+ * Average performance O(n)
+ * Worst-case space complexity O(min(n, m)), where n is the length of the string
+ * and m is the size of the character set.
+ *
+ * @author  (https://github.com/Chiefpatwal)
+ */
+public final class LongestSubstringWithoutRepeatingCharacters {
+
+    // Prevent instantiation
+    private LongestSubstringWithoutRepeatingCharacters() {
+    }
+
+    /**
+     * This method finds the length of the longest substring without repeating characters.
+     *
+     * @param s is the input string
+     * @return the length of the longest substring without repeating characters
+     */
+    public static int lengthOfLongestSubstring(String s) {
+        int maxLength = 0;
+        int left = 0;
+        HashSet<Character> charSet = new HashSet<>();
+
+        for (int right = 0; right < s.length(); right++) {
+            // If the character is already in the set, remove characters from the left
+            while (charSet.contains(s.charAt(right))) {
+                charSet.remove(s.charAt(left));
+                left++;
+            }
+            charSet.add(s.charAt(right));
+            maxLength = Math.max(maxLength, right - left + 1);
+        }
+
+        return maxLength;
+    }
+}