Create README.md

Author: jaygajera17

README.md

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+# gfg_string_question
+https://www.geeksforgeeks.org/top-50-string-coding-problems-for-interviews/
+
+## Importance
+- playing with characters test logic of programmer very well
+- understanding of C++ Standard Template Library(STL) also increase
+
+## Key Points
+### ascii value:-
+- 48 to 57 - Number
+- 65 to 90 - uppercase [(s[0]>='a'&&s[0]<='z']
+- 97 to 122 - lowercase [(s[0]>='A'&&s[0]<='Z']
+-lowercase = lowercase + 32
+
+
+### inbuilt Method:-
+
+- toupper:-  for(int i=0;i<s.length();i++)
+             ans=ans+(char)tolower(s[i]);
+- tolower:- (char)toupper(s[i]);
+
+
+- Transform:- transform(s.begin(), s.end(), s.begin(), ::tolower);
+           transform(s.begin(), s.end(), s.begin(), ::toupper);
+
+- Find:-  s1.find(s2[i]):- find s2[i] in s1 
+
+