upGrad String

The next problem will help you in developing a strong sense of dictionary key and value. For the purpose of this question, we will define something called an upGrad string. Note that this definition is not valid outside this question.

A string is an upGrad string if the frequency of its characters is something like 1, 2, 3, 4, ... i.e., a character appears only once, another appears twice, another appears thrice and so on.

For example string '$yrr$ssrsr' is an upGrad string since the frequency of y:1, $:2, s:3, r:4, however, string '$yrr$ssrsr%' will not be an upGrad string since it has two characters (y and %) with frequency 1. The frequency of characters should be of the form 1, 2, 3, 4, 5... only. Given a string, can you determine if the string is upGrad string or no? 

Here Sajan explains how to code this problem. The key takeaway from the last two problems is to learn how to iterate on dictionary keys and values.