Added Longest substring without duplication in Java/strings.

strings/LongestSubstringWithoutDuplication.java

@@ -0,0 +1,28 @@
+import java.util.*;
+
+public class LongestSubstringWithoutDuplication {
+
+  public static void main(String[] args) {
+    // Testcases
+    assert longestUniqueSubstring("lmnoppqrrsss").equals("lmnop");
+    assert longestUniqueSubstring("falafel").equals("lafe");
+    assert longestUniqueSubstring("krishnamachari").equals("krishnam");
+  }
+
+  // Finds the longest substring of a string having unique characters
+  public static String longestUniqueSubstring(String str) {
+    if (str.equals("")) return "";
+    HashMap<Character, Integer> lastSeenAt = new HashMap<>();
+    int[] longest = new int[] { 0, 1 };
+    int start = 0;
+    for (int i = 0; i < str.length(); i++) {
+      char c = str.charAt(i);
+      if (lastSeenAt.containsKey(c)) start =
+        Math.max(start, lastSeenAt.get(c) + 1);
+      if (longest[1] - longest[0] < i + 1 - start) longest =
+        new int[] { start, i + 1 };
+      lastSeenAt.put(c, i);
+    }
+    return str.substring(longest[0], longest[1]);
+  }
+}