You're given a sum of several numbers like 3+2+1. The numbers are always , , or , separated by plus signs. Your task: rewrite the sum with the numbers in non-decreasing order.
For example, 3+2+1 becomes 1+2+3. This problem tests string parsing and sorting. You'll need to extract the numbers from the input string, sort them, then reconstruct the output with plus signs between them.
It's your first taste of working with strings beyond simple input and output. Before reading the solution, think about how you'd extract just the numbers. What character separates them? How can you skip over the plus signs and collect only the digits?