isShuffle dynamic programming

gistfile1.py

def isShuffle2(x,y,z):
    m = len(x)
    n = len(y)
    r = len(z)
    
    S = {}
    
    #inicializace pole
    for i in range(-1,r+1):
        for j in range(-1,r+1):
            S[(i,j)] = False
    S[(0,0)] = True

        
    for i in range(0,r+1):
        for j in range(0,r+1):
            
            if (i+j > r):
                continue
            
            if (i == 0 and j == 0):
                S[(0,0)] = True
            elif (x[(i-1) % m] == z[i+j-1] and y[(j-1) % n] != z[i+j-1]):
                S[(i,j)] = S[(i-1,j)]
            elif (x[(i-1) % m] != z[i+j-1] and y[(j-1) % n] == z[i+j-1]):
                S[(i,j)] = S[(i,j-1)]
            elif (x[(i-1) % m] == z[i+j-1] and y[(j-1) % n] == z[i+j-1]):
                S[(i,j)] = S[(i,j-1)] or S[(i-1,j)]
                             
    for i in range(0,r+1):
        for j in range(0,r+1):
            if i+j == r and S[(i,j)]:
                return True
            
    return False
    
print isShuffle2("slovo", "word", "slwoovosrdlworovodwordwors")