list manipulation - How to rebuild original data from a histogram?

I have data in the following form. I have a list of values and a list of frequencies that indicate how often each value has occurred.

Example list of values: {5,7,4}

Example list of frequencies: {1,2,3}

I would like to obtain the original data from which such a histogram was generated. Any suggestions on how to do this with Mathematica ?

Example: {5,7,7,4,4,4}

Answer

Without looking at performance, but only on understanding: First, you create a function f which takes a value and a count and which reproduces the value exactly count times. In the simplest case

f[val_, count_] := ConstantArray[val, count]

and you can call f[3,4] to get {3,3,3,3}. Now, you combine your input arrays so that you can call f directly for each pair. For this, you can use MapThread. To create you final result, you have to Flatten the output:

Flatten[MapThread[f, {{5, 7, 4}, {1, 2, 3}}]]

This all can of course be combined into one call

vals = {5, 7, 4};
counts = {1, 2, 3};


Flatten[MapThread[ConstantArray, {vals, counts}]]

or

ConstantArray @@@ Transpose[{vals, counts}] // Flatten

or (to simplify kgulers approach)

Inner[ConstantArray, vals, counts, Join]

and many more