Here's your approach:
Read the input string.
Create an empty vector to store digits.
Loop through each character in the string.
If the character is a digit, add it to the vector.
Sort the vector using sort().
Print the first digit.
For each remaining digit, print a plus sign followed by the digit.
Done. The solution uses filtering, sorting, and formatted output. You separate concerns: first extract digits, then sort them, then format the result. Each step is simple on its own.