Ositus

string = input()
length = len(string)


symbol = ['', string[0]]

index_of_l = [0, 1] 
index_of_ntl = [0, 0] 

number_for_l = [0, 1]
number_for_ntl = [0, 0] 


count = 2

rightest_ntl = 0



preceding = 1

prime_modulus = 1000000007


for i in range(1, length):


    if string[i-1] == string[i]:

        index_of_ntl[preceding] = index_of_l[preceding]
        index_of_l[preceding] = i
        number_for_ntl[preceding] = number_for_l[preceding]
        continue

    for j in range(1, count):
            
        if string[i] == symbol[j]:


            
            if index_of_ntl[rightest_ntl] > index_of_l[j]:
                index_of_ntl[j] = index_of_l[j]
                index_of_l[j] = i
                number_for_ntl[j] = number_for_l[j]

                
                number_for_l[j] = 2 * number_for_l[preceding]

            elif index_of_l[rightest_ntl] <= index_of_l[j]:
                index_of_ntl[j] = index_of_l[j]
                index_of_l[j] = i
                number_for_ntl[j] = number_for_l[j]

                number_for_l[j] = 2 * number_for_l[preceding] - number_for_ntl[j]
                
                rightest_ntl = j
                
            else:
                index_of_ntl[j] = index_of_l[j]
                index_of_l[j] = i
                number_for_ntl[j] = number_for_l[j]
                
                number_for_l[j] = 2 * number_for_l[preceding] - number_for_ntl[j] + number_for_ntl[rightest_ntl]
                number_for_l[rightest_ntl] -= number_for_ntl[j]
                
                rightest_ntl = j
                    
            if not 0 <= number_for_l[j] < prime_modulus:
                number_for_l[j] %= prime_modulus
                    
            preceding = j
                    
            break


    else:   
        symbol += [string[i]]
            
        index_of_l += [i]
        index_of_ntl += [0]
        
        number_for_l += [2 * number_for_l[preceding]]

        count += 1
        
        if number_for_l[count - 1] > prime_modulus:
            number_for_l[count - 1] %= prime_modulus
        number_for_ntl += [0]
        
        preceding = count - 1


    

print(number_for_l[preceding])

Test 1

Group: 1, 2, 3

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3

Verdict: WRONG ANSWER

Test 4

Group: 1, 2, 3

Verdict: WRONG ANSWER

Test 5

Group: 2, 3

Verdict: WRONG ANSWER

Test 6

Group: 3

Verdict: WRONG ANSWER

Test 7

Group: 3

Verdict: WRONG ANSWER