Find smallest permutation of given number

Given a long integer, return the smallest(magnitude) integer permutation of that number.
Examples: 
 

Input : 5468001
Output : 1004568

Input : 5341
Output : 1345

Question Source : GE digital Interview Experience | Set 6
We have already discussed a solution in below post.
Smallest number by rearranging digits of a given number
In this post, a different approach is discussed.
Approach : As number is long, store the number as string, sort the string, if there is no leading zero, return this string, if there is any leading zero, swap first element of string with first non-zero element of string, and return the string.
Below is the implementation of above approach : 
 

Output: 
 

1004568

Optimization : 
Since character set is limited ('0' to '9'), we can write our own sort method that works in linear time (by counting frequencies of all characters)