Update SHORT_NOTES_on_STL.md

Author: raihankhan

SHORT_NOTES_on_STL.md

@@ -2,5 +2,23 @@
 
 ## map and unordered_map
 map is a very important STL used in competitive programming. Map is implemented using Red-Black Tree. Maps store values in the form of a combination of a key value and a mapped value.
+Declaration:
+map<int,int>mm ;
+unordered_map<int,int>mm ;
 
-### find()
+### mm.find(k)
+Returns an iterator of the specified key value if it's been found in the map or returns an iterator to mm.end() if element not found.
+
+map<int,int>mm;
+mm[4]=9;
+mm[3]=2;
+
+auto it=mm.find(4);
+
+if(it==mm.end())
+cout << "element not found" << endl;
+else cout << it->first << " " << it->second << endl;  // prints- 4 9
+
+complexity:
+for map complexity is logarithmic.
+for unordered_map complexity is constant(average), linear(worst).