def C1(i, j):
    x = " " + i_word
    y = " " + j_word
    
    if i == 0 or j == 0:
        return 0
    elif (i > 0 and j > 0) and x[i] == y[j]:
        return C1(i-1, j-1) + 1
    elif (i > 0 and j > 0) and x[i] != y[j]:
        print(i, j)
        return max(C1(i-1, j), C1(i, j-1))

def C2(i, j, k):
    x = " " + i_word
    y = " " + j_word
    z = " " + k_word
    
    if i == 0 or j == 0 or k == 0:
        return 0
    elif (i > 0 and j > 0 and k > 0) and x[i] == y[j] == z[k]:
        return C2(i-1, j-1, k-1) + 1
    elif (i > 0 and j > 0 and k > 0) and x[i] != y[j] != z[k]:
        return max(C2(i-1, j, k), C2(i, j-1, k), C2(i, j, k-1))

我試著用這樣的方法寫出了求兩個字串和求三個字串LCS的程式碼，兩個字串的可以正常運行，但不知道為什麼三個字串的會回傳出None來，希望各位可以為小弟解答