There are just two ways to insert four plus or minus signs between the digits 123456789 to yield a sum of 1:
1 + 23 - 45 - 67 + 89 = 1
1 - 23 + 45 + 67 - 89 = 1
Now, see if you can insert four plus or minus signs in the sequence 987654321 to make a sum of 1. There is only one solution.