Restructure preparation section

algorithms/string.md

@@ -24,15 +24,12 @@ String
 - Given a string, find the longest substring without repeating characters. For example, for string `'abccdefgh'`, the longest substring is `'cdefgh'`.
   - [Source](http://blog.gainlo.co/index.php/2016/10/07/facebook-interview-longest-substring-without-repeating-characters/)
 - Given a string, return the string with duplicate characters removed.
-- How many string representations are there for an integer where `a->1, b->2, ... z->26`.
-  - E.g. `126` can be `'az'` or `'abf'`.
 - Write a function that receives a regular expression (allowed chars = from `'a'` to `'z'`, `'*'`, `'.'`) and a string containing lower case english alphabet characters and return `true` or `false` whether the string matches the regex.
   - E.g. `'ab*a'`, `'abbbbba'` => `true`.
   - E.g. `'ab*b.'`, `'aba'` => `true`.
   - E.g. `'abc*'`, `'acccc'` => `false`.
 - Given a rectangular grid with letters, search if some word is in the grid.
 - Given two strings representing integer numbers (`'123'` , `'30'`) return a string representing the sum of the two numbers: `'153'`.
-- Given a really big file with a lots of Facebook posts, find the ten most-used words.
 - A professor wants to see if two students have cheated when writing a paper. Design a function `hasCheated(String s1, String s2, int N)` that evaluates to `true` if two strings have a common substring of length `N`.
   - Follow up: Assume you don't have the possibility of using `String.contains()` and `String.substring()`. How would you implement this?
 - Print all permutations of a given string.