• Reverse a String
• check if a string is palindrome
• Count the Number of Words in a String
• Check if a String Contains a Substring
• Convert a String to Uppercase or Lowercase
• Find the First Non-Repeating Character
• Remove Duplicate Characters from a String
• Find the Most Frequent Character in a String
• Check if Two Strings are Anagrams
• Check if Two Strings are Rotations of Each Other
• Reverse Words in a Sentence
• Convert an Integer to a String Without Using strconv
• Find All Substrings of a Given String

hard : do it always

• Find the Longest Common Prefix Among a Set of Strings
• Valid parentheses