递归寻路算法始终返回None

def recursive_search(grid, start, end):
    if start == end:
        return [start]
    # 探索四个方向
    for dx, dy in [(0, 1), (1, 0), (0, -1), (-1, 0)]:
        x, y = start[0] + dx, start[1] + dy
        if 0 <= x < len(grid) and 0 <= y < len(grid[0]) and grid[x][y] != 'obstacle':
            path = recursive_search(grid, (x, y), end)
            if path:
                return [start] + path
    return None

def recursive_search(grid, start, end, visited=None):
    if visited is None:
        visited = set()
    if start == end:
        return [start]
    visited.add(start)
    for dx, dy in [(0, 1), (1, 0), (0, -1), (-1, 0)]:
        x, y = start[0] + dx, start[1] + dy
        if 0 <= x < len(grid) and 0 <= y < len(grid[0]) and grid[x][y] != 'obstacle' and (x, y) not in visited:
            path = recursive_search(grid, (x, y), end, visited)
            if path:
                return [start] + path
    visited.remove(start)
    return None