The strategy I would use is the following:
Goal is to get $100$ as sum. Among the digits $123456789$, pick and choose the sum close to $100$, such as $89$. Therefore I would attempt to get a value of $11$ from $1234567$ using different combinations.
When you start working on a smaller sum now (sort of like divide and conquer), you may get the desired result. (Of course there is no specific algorithm).
In order to get $11$, I have
$$(1\times 23)-4+5-6-7 = 11$$
$$(1-2+3-4+5)\times 6 -7= 11$$
$$123-45-67 = 11$$
Therefore
$$(1\times 23)-4+5-6-7+89 = 100$$
$$(1-2+3-4+5)\times 6 - 7+89=100$$
$$123-45-67+89=100$$
${\bf{Adding}}$ ${\bf{more}}$ to it: If we look at $78+9 = 87$ and instead of $89$, we seek the remaining $13$ to be derived from $123456$, and one way to get that is
$$6+5+4-3+2-1=13$$
Therefore
$$78+9+6+5+4-3+2-1=100$$