Ositus

from collections import deque
from string import ascii_lowercase

s = "-" + input() + "x"


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

minimum = end + 1


seen = {ascii_lowercase[i]: end+1 for i in range(26)}

last = {i: -1 for i in range(end+2)}
distances = deque()

prevdist = 9999999

for i in range(end, 0, -1):
    minimum = min(minimum, seen[s[i]])

    if minimum < prevdist:
        distances.insert(0, minimum)
        prevdist = minimum

    # maxnb[i] = seen[s[i]]
    maxnb[i] = minimum
    seen[s[i]] = i

    last[maxnb[i]] = max(last[maxnb[i]], i)


def getsum(a, b):
    return sumtable[b] - sumtable[a-1]


# i+1 indeksointi
sumtable = [0, 1]


start = 1
mindist = distances.popleft()
for i in range(2, end+1):
    # check
    if i > mindist:
        start = last[mindist] + 1
        mindist = distances.popleft()

    sumtable.append(sumtable[i-1] + getsum(start, i-1))


MOD = 10**9 + 7


print((getsum(end, end)) % MOD)
# print(int(getval(end, -1) % MOD))

Test 1

Group: 1, 2, 3

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3

Verdict: ACCEPTED

Test 4

Group: 1, 2, 3

Verdict: ACCEPTED

Test 5

Group: 2, 3

Verdict: ACCEPTED

Test 6

Group: 3

Verdict: ACCEPTED

Test 7

Group: 3

Verdict: RUNTIME ERROR