Task $2$

Write a function solution that, given a string S consisting of N lowereare letters returns a string that only has letters that occur once in the orisingl string, in the order that they appear on the original string.

Examples:

Given $S=$ "codility", the function should return "codity".

Given $S=$ "background" the function should retum "background.

Given S $=$ "marmalade" the function should return "flde".

Given $S=$ "peep" the function should return $"($blank string$).$

Write an efficient algorithm for the following assumptions:

N is an integer within the range $[1.\mathrm{.}300,000]$;

string S is made only of lowercase letters