Leetcode 741. Cherry Pickup

Given an n×n grid with cherries (1), empty cells (0) and thorns (-1), find the maximum cherries collectible by going from (0,0) to (n-1,n-1) moving only right/down and then returning moving only left/up, where cherries are picked once and thorns block traversal. The core challenge is handling the round-trip overlap—typically modeled as two simultaneous forward paths (or DP over step and two positions) to maximize cherries without double-counting and while avoiding blocked cells.