Get Sequence Sum

Problem Overview

• Compute the sum of a number sequence that starts at i, increases by 1 to j, then decreases by 1 to k, as described.
• Input: three integers i, j, k; Output: a long integer sum. Constraints: -1e8 ≤ i, j, k ≤ 1e8 and i, k ≤ j.
• Domain: arithmetic series and sequence processing in math-focused programming.
• Source: Ibm interviews; a classic coding interview problem and interview question.