A* Search

1g[start] = 0;  // Cost from start is zero2open.add(new int[]{h(start), start});  // Seed open set by f = h3while (!open.isEmpty()) {  // Until open set is empty4  int node = open.poll()[1];  // Pop lowest-f cell5  if (settled[node]) continue;  // Skip stale duplicates6  settled[node] = true;  // Lock in this cell7  if (node == goal) return buildPath();  // Goal found — trace path8  for (int neighbor : neighbors(node)) {  // Scan passable neighbors9    int tentativeG = g[node] + cost(neighbor);  // Cost to reach via node10    if (tentativeG < g[neighbor]) {  // Found a cheaper route11      g[neighbor] = tentativeG; parent[neighbor] = node;  // Relax and record parent12      open.add(new int[]{tentativeG + h(neighbor), neighbor});  // Queue by f = g + h13    }14  }15}16 17int h(int cell) {  // Manhattan distance to goal18  int r = cell / COLS, c = cell % COLS;19  int gr = goal / COLS, gc = goal % COLS;20  return Math.abs(r - gr) + Math.abs(c - gc);21}22 23List<Integer> neighbors(int cell) {  // Passable 4-neighbors24  int r = cell / COLS, c = cell % COLS;25  int[][] dirs = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};26  List<Integer> out = new ArrayList<>();27  for (int[] d : dirs) {28    int nr = r + d[0], nc = c + d[1];29    if (nr < 0 || nr >= ROWS || nc < 0 || nc >= COLS) continue;30    int nbr = nr * COLS + nc;31    if (!walls.contains(nbr)) out.add(nbr);32  }33  return out;34}35 36int cost(int cell) {  // Step cost to enter a cell37  return weights.contains(cell) ? WEIGHT_COST : 1;38}39 40List<Integer> buildPath() {  // Walk parent back from goal41  List<Integer> path = new ArrayList<>();42  for (int at = goal; at != -1; at = parent[at]) path.add(at);43  Collections.reverse(path);44  return path;45}