Leetcode 2400. Number of Ways to Reach a Position After Exactly k Steps

Count the number of length-k sequences of ±1 moves on an infinite number line that take you from startPos to endPos — equivalently, choose how many right vs left steps produce the required net displacement. Return the count modulo 10^9+7 (zero if |endPos−startPos|>k or parity mismatches); typical solutions use combinatorics (binomial coefficients) or DP since k ≤ 1000.