The Knight and the Maze

# http://puzzling.stackexchange.com/q/38324/40

def knight():
    m = ["111111111110110100001000010000000001000100",
         "000000000000010101000001000000000100010001",
         "011111111111100001000100011001010010100010",
         "000000000000000010000000000101010000000010",
         "011010101001100010011001000100000010101000",
         "000000000000100001100100000100011000000000",
         "111011011101100000000001100101001001000111",
         "000000000000000100000000111001001001000100",
         "111111111110100000101100000000000100010110",
         "000000000000000010100001000000000100010010",
         "110110111111100010100100010101000000000000",
         "100000000000111100010100010101010110011100",
         "110111111110100100010001000000110001110101"]
    a = []
    w = len(m[0])
    h = len(m)
    MAX = w*h+1
    for i in range(h):
        a.append([])
        for j in range(w):
            a[i].append([-1,MAX][int(m[i][j])])
    a[0][0] = 0
    q = [(0,0)]
    while len(q) > 0:
        (x,y),q = q[0],q[1:]
        for dx in [-2,-1,1,2]:
            for dy in [-2,-1,1,2]:
                if abs(dx*dy) != 2:
                    continue
                xx, yy = x+dx, y+dy
                if (xx>=0 and xx<w and yy>=0 and yy<h and
                      a[yy][xx] != -1 and a[yy][xx]>a[y][x]+1):
                    a[yy][xx] = a[y][x]+1
                    q.append((xx,yy))
    for i in range(h):
        o = ""
        for j in range(w):
            c = a[i][j]
            if c == -1:
                o = o + "."  # blue cell (off-limits)
            elif c == MAX:
                o = o + "-"  # yellow, but still unreachable
            else:
                if c > 25:
                    c += 6
                o = o + chr(c+65)
        print o

knight()