Your code will result in Integer Overflow because of this

Also, your code will fail for numbers with trailing zeros as such:

Test Failed
  Expected string length 42 but was 27. Strings differ at index 7.
  Expected: "6000000 + 800000 + 70000 + 3000 + 700 + 20"
  But was:  "600000080000070000300070020"

with the output panel displaying:

6873720
num 20
num 700
num 3000
num 70000
num 800000
num 6000000