Given a Salesforce workflow configuration string config consisting of digits (0-9) and placeholders (?), determine the number of ways the placeholders can be replaced with digits (0-9) such that no two adjacent workflow steps (digits) are the same.
The workflow configuration string consisting of digits (0-9) and placeholders (?).
The total number of valid configurations modulo (10^9 + 7).
Given the workflow configuration string config = "1?3?":
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