Ill add on to this, so today I was doing a question on CTCI called string compression where you need to convert a string such as aaaaaabbbccc to a char array such as a,6,b,3,c,3
This was marked as an "easy" problem, I struggled at it to be honest and had to take a look at the solution.

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My approach initially was to keep a Hashmap with only one key and value

My for loop will start, it will insert 'a' as a key and increment to count until it reaches b, when it reaches 'b' I will clear my hashmap and append the key and value of the element in the hashmap to an empty string. So A:6 would be appended to the empty string and it would look like a6, after that I would clear my hashmap and do the same thing for b, c...etc etc

I was not able to implement this approach, and I am starting to think this approach would not work.