Find Number of Valid Pairs

Problem Overview

• Count pairs of indices (i, j) with i < j where a[i] − b[j] equals a[j] − b[i].
• Input: two same-length lists a and b; Output: the number of valid pairs; constraints not specified.
• Context: balancing cross-differences when comparing two datasets or signals.
• From Roblox interviews; a classic coding interview problem and interview question.