Count Four-Digit Codes with Sum S

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Problem Overview

  • Compute how many four-digit codes (0000–9999) have digits summing to S.
  • Input: integer S; Output: count of such codes.
  • Constraints: S ∈ [0..36]; leading zeros are allowed.
  • Real-world context: counting valid PIN combinations under a digit-sum rule.
  • From Google interviews; a classic coding interview problem and interview question.

Example

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Input Variables
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