glowing's job puzzle

GLOW_PUZZLE.md

seeing this puzzle in glowing's jobs page

https://glowing.com/jobs/mobiledeveloper#mtop

here's my solution

glow_puzzle.py

S = ' RBB' 'RRBB' 'RRBB' 'RRBB'
E = 'RRBB' 'RBBB' 'R RB' 'RRBB'
T = ' BRB' 'BRBR' 'RBRB' 'BRBR'

def solve_puzzle(A, B):
    visited = set([A])
    tried = [(A, '')]
    pos = 0
    while True:
        end = len(tried)
        for j in range(pos, end):
            C, p = tried[j]
            i = C.index(' ')
            for d in 'LRUD':
                N = list(C)
                if d == 'L' and i not in [0, 4, 8, 12]:
                    N[i], N[i-1] = N[i-1], N[i]
                elif d == 'R' and i not in [3, 7, 11, 15]:
                    N[i], N[i+1] = N[i+1], N[i]
                elif d == 'U' and i not in [0, 1, 2, 3]:
                    N[i], N[i-4] = N[i-4], N[i]
                elif d == 'D' and i not in [12, 13, 14, 15]:
                    N[i], N[i+4] = N[i+4], N[i]
                else:
                    continue
                N = ''.join(N)
                if N not in visited:
                    if N == B:
                        return p+d
                    visited.add(N)
                    tried.append((N, p+d))
        pos = end

print solve_puzzle(S, E)
print solve_puzzle(S, T)