Added Java solution

Author: Kyrylo-Ktl

Java/Longest Substring Without Repeating Characters.java

@@ -0,0 +1,19 @@
+class Solution {
+
+    /**
+     * Time:   O(n)
+     * Memory: O(n)
+     */
+    public int lengthOfLongestSubstring(String s) {
+        int[] nextIndex = new int[128];
+        int longest = 0, left = 0;
+
+        for (int right = 0; right < s.length(); right++) {
+            left = Math.max(nextIndex[s.charAt(right)], left);
+            longest = Math.max(longest, right - left + 1);
+            nextIndex[s.charAt(right)] = right + 1;
+        }
+
+        return longest;
+    }
+}