Get Minimum Cost

Problem Overview

• Find the minimum total cost to buy exactly m items from n types; the j-th item of type i costs a[i] + (j-1)*b[i].
• Input: arrays a[n], b[n] and integer m. Output: a long integer minimum cost.
• Context: Amazon Go Store pricing with infinite stock per type and increasing unit cost.
• Constraints: 1≤n,m≤1e5; 1≤a[i],b[i]≤1e5.
• Amazon coding interview problem; a common interview question from Amazon interviews.