We need to rearrange the three numbers, given as a string like 3+2+1, and print the sum expression with the values sorted
in non-decreasing order. So 3+2+1 becomes 1+2+3. There's no tricks here, the string is non-empty and does
not have spaces, each value will only be one of $1$, $2$, or $3$, and they are separated by the character literal +.