Target Sum

Write `find_target_sum_ways(nums, target)` counting the ways to put a `+` or `-` in front of each number so the expression equals the target. find_target_sum_ways([1,1,1,1,1], 3) -> 5 Brute force is 2^n. Memoising on (index, running total) collapses the repeated states. The neat reduction: if P is the positive subset then P - (total - P) = target, so P = (total + target) / 2 — which turns this into counting subsets with a given sum, and immediately shows when the answer is 0 for parity reasons.