Ositus

from collections import defaultdict

s = input()

end = len(s)
maxnb = [end for _ in range(end)]
maxnb.append(-1)

minimum = end

def default():
    return minimum

def def2():
    return -1

seen = defaultdict(default)

for i in range(end-1, -1, -1):
    minimum = seen[s[i]]
    maxnb[i] = seen[s[i]]
    seen[s[i]] = i

val = defaultdict(def2)
val[0] = 1

def getval(cur, prev):
    if maxnb[cur] < prev:
        return 0

    if val[cur] != -1:
        return val[cur]

    ans = 0

    for i in range(0, cur):
        ans += getval(i, cur)

    val[cur] = ans

    return ans

print(int(getval(end, -1) % (10**9 + 7)))

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: TIME LIMIT EXCEEDED