Ositus

s = input()
r = 0
vr = 0
m = {}

for c in s:
    try:
        m[c]
    except KeyError:
        m[c] = [0, 0]

    if r == 0:
        r = 1
    else:
        r *= 2
        cvr = vr - m[c][1]
        cvr = max(0, cvr)
        cvr = m[c][0] - cvr
        cvr = max(0, cvr)
        r -= cvr
        vr += cvr
    m[c] = [r, vr]

r %= 1000000007
print(r)

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: TIME LIMIT EXCEEDED