Added Problem 3

Author: mkhuzaima

0003_Longest_Substring_Without_Repeating_Characters.java

@@ -0,0 +1,27 @@
+// id: 3
+// Name: Longest Substring Without Repeating Characters
+// link: https://leetcode.com/problems/longest-substring-without-repeating-characters/
+// Difficulty: Medium
+
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        int max = 0;
+
+        Map<Character, Integer> lastOccurrence = new HashMap<>();
+
+        int prev = 0;
+        for (int current = 0; current < s.length(); current++) {
+            // if last occurrence of current is before `prev`, then greater will be considered.
+
+            // like abba
+            // at index 3, last occurrence of a is 0, but 1 will be considered. 
+          
+            // there will be distinct characters from in [prev, current]
+            prev = Math.max(lastOccurrence.getOrDefault(s.charAt(current), prev), prev);
+
+            max = Math.max(current-prev+1, max);
+            lastOccurrence.put(s.charAt(current), current+1);
+        }
+        return max;
+    }
+}