Minimum Moves to Move a Box to Their Target Location

from collections import deque

class Solution:

    def is_valid(self, ri, ci, rn, cn):
        if ri < 0 or ri >= rn or ci < 0 or ci >= cn:
            return False

        return True

    def is_reachable(self, start_ri, start_ci, target_ri, target_ci, box_ri, box_ci, grid):

        rn = len(grid)
        cn = len(grid[0])

        if not self.is_valid(target_ri, target_ci, rn, cn) or grid[target_ri][target_ci] == '#':
            return False

        q = deque()
        q.append((start_ri, start_ci))
        visited = set()
        visited.add((start_ri, start_ci))

        moves = [(-1, 0), (0, 1), (1, 0), (0, -1)]

        while q:

            ri, ci = q.popleft()

            if ri == target_ri and ci == target_ci:
                return True

            for kr, kc in moves:

                xri = ri + kr
                xci = ci + kc

                if self.is_valid(xri, xci, rn, cn) and \
                   grid[xri][xci] != '#' and \
                   (xri != box_ri or xci != box_ci) and \
                   (xri, xci) not in visited:
                    q.append((xri, xci))
                    visited.add((xri, xci))

        return False

    def minPushBox(self, grid: List[List[str]]) -> int:

        rn = len(grid)
        cn = len(grid[0])

        for ri in range(rn):
            for ci in range(cn):
                if grid[ri][ci] == 'T':
                    t_ri, t_ci = ri, ci
                elif grid[ri][ci] == 'B':
                    b_ri, b_ci = ri, ci
                elif grid[ri][ci] == 'S':
                    s_ri, s_ci = ri, ci

        q = deque()
        q.append((b_ri, b_ci, s_ri, s_ci, 0))
        visited = set()
        visited.add((b_ri, b_ci, s_ri, s_ci))

        moves = [(-1, 0), (0, 1), (1, 0), (0, -1)]

        while q:

            b_ri, b_ci, s_ri, s_ci, lvl = q.popleft()

            if b_ri == t_ri and b_ci == t_ci:
                return lvl

            for kr, kc in moves:

                next_b_ri = b_ri + kr
                next_b_ci = b_ci + kc

                s_push_ri = b_ri + -1 * (kr)
                s_push_ci = b_ci + -1 * (kc)

                if self.is_valid(next_b_ri, next_b_ci, rn, cn) and \
                   grid[next_b_ri][next_b_ci] != '#' and \
                   self.is_reachable(s_ri, s_ci, s_push_ri, s_push_ci, b_ri, b_ci, grid) and \
                   (next_b_ri, next_b_ci, s_push_ri, s_push_ci) not in visited:
                    q.append((next_b_ri, next_b_ci, s_push_ri, s_push_ci, lvl+1))
                    visited.add((next_b_ri, next_b_ci, s_push_ri, s_push_ci))

        return -1