Create longest-substring-without-repeating-characters.cs

3-longest-substring-without-repeating-characters/longest-substring-without-repeating-characters.cs

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+public class Solution {
+    public int LengthOfLongestSubstring(string s) {
+        var set = new HashSet<char>();
+        int i = 0, longest = 0;
+        for (int j = 0; j < s.Length; ++j) {
+            char currentChar = s[j];
+            while (set.Contains(currentChar)) {
+                set.Remove(s[i++]);
+            }
+            set.Add(currentChar);
+            longest = Math.Max(longest, j - i + 1);
+        }
+        return longest;
+    }
+}