Add C++ 19 20 Solutions

Author: jinshendan

Problems/20-Optimal-Partition-of-String/Optimal-Partition-of-String.cpp

@@ -0,0 +1,22 @@
+class Solution {
+public:
+    int partitionString(string s) {
+        int ans = 0;
+        unordered_set<char> st;
+        for (auto& c: s)
+        {
+            if (st.find(c) == st.end())
+            {
+                st.insert(c);
+            }
+            else
+            {
+                st.clear();
+                st.insert(c);
+                ans++;
+            }
+        }
+        if (!st.empty()) ans++;
+        return ans;
+    }
+};