Move all solutions into separate directory (cleanup)

the beginning

solutions/longest-substring-without-repeating-characters/Solution.java

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+// Accepted
+// Runtime: 4 ms
+// Memory Usage: 39.1 MB
+// Submitted: January 13th, 2021
+
+class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        int max = 0;
+        // Map stores the character's (latest_index + 1)
+        Map<Character, Integer> map = new HashMap<Character, Integer>();
+        
+        int i = 0;
+        for (int j = 0; j < s.length(); j++) {
+            char c = s.charAt(j);
+            
+            // If we've already seen this character, move up i (if needed)
+            if (map.containsKey(c))
+                i = Math.max(map.get(c), i);
+            
+            max = Math.max(max, j - i + 1);
+            map.put(c, j + 1);
+        }
+        
+        return max;
+    }
+}