Buggy python submission (verdict: Time Limit Exceeded):
from collections import Counter, defaultdict
N = int(input())
S = list(input())
Q = int(input())

l = 2**((N-1).bit_length())
data = [Counter([]) for i in range(2*l)]
for i in range(N):
    data[l+i][S[i]] = 1
for i in range(l-1,0,-1):
    data[i] = data[2*i] + data[2*i+1]

for cnt in range(Q):
    a,b,c = input().split()
    a = int(a)
    b = int(b)
    if a == 1:
        if S[b-1] == c:
            continue
        oldc = S[b-1]
        S[b-1] = c
        n = l + b - 1
        while n > 0:
            data[n][oldc] -= 1
            data[n][c] += 1
            n = n // 2
    else:
        c = int(c)
        i = b + l -1
        j = c + l
        temp = Counter()
        while j-i > 0:
            if i & 1:
                temp += data[i]
                i += 1
            if j & 1:
                temp += data[j-1]
                j -= 1
            i //= 2
            j //= 2
        print(len(temp))



Fixed submission:
N = int(input())
S = list(input())
Q = int(input())

l = 2**((N-1).bit_length())
#data = [Counter([]) for i in range(2*l)]
data = [0] * (2*l)

str2bin = lambda x : 1 << (ord(x) - ord('a'))
    
for i in range(N):
    data[l+i] = str2bin(S[i])
for i in range(l-1,0,-1):
    data[i] = data[2*i] | data[2*i+1]

for cnt in range(Q):
    a,b,c = input().split()
    a = int(a)
    b = int(b)
    if a == 1:
        n = l + b - 1
        data[n] = str2bin(c)
        n //= 2
        while n > 0:
            data[n] = data[2*n] | data[2*n+1]
            n = n // 2
    else:
        c = int(c)
        i = b + l -1
        j = c + l
        res = 0
        while j-i > 0:
            if i & 1:
                res |= data[i]
                i += 1
            if j & 1:
                res |= data[j-1]
                j -= 1
            i //= 2
            j //= 2
        print(bin(res).count('1'))

Buggy python submission (verdict: Time Limit Exceeded):
from collections import Counter, defaultdict
N = int(input())
S = list(input())
Q = int(input())

l = 2**((N-1).bit_length())
data = [Counter([]) for i in range(2*l)]
for i in range(N):
    data[l+i][S[i]] = 1
for i in range(l-1,0,-1):
    data[i] = data[2*i] + data[2*i+1]

for cnt in range(Q):
    a,b,c = input().split()
    a = int(a)
    b = int(b)
    if a == 1:
        if S[b-1] == c:
            continue
        oldc = S[b-1]
        S[b-1] = c
        n = l + b - 1
        while n > 0:
            data[n][oldc] -= 1
            data[n][c] += 1
            n = n // 2
    else:
        c = int(c)
        i = b + l -1
        j = c + l
        temp = Counter()
        while j-i > 0:
            if i & 1:
                temp += data[i]
                i += 1
            if j & 1:
                temp += data[j-1]
                j -= 1
            i //= 2
            j //= 2
        print(len(temp))



Fixed submission:
N = int(input())
S = list(input())
Q = int(input())

l = 2**((N-1).bit_length())
#data = [Counter([]) for i in range(2*l)]
data = [0] * (2*l)

str2bin = lambda x : 1 << (ord(x) - ord('a'))
    
for i in range(N):
    data[l+i] = str2bin(S[i])
for i in range(l-1,0,-1):
    data[i] = data[2*i] | data[2*i+1]

for cnt in range(Q):
    a,b,c = input().split()
    a = int(a)
    b = int(b)
    if a == 1:
        n = l + b - 1
        data[n] = str2bin(c)
        n //= 2
        while n > 0:
            data[n] = data[2*n] | data[2*n+1]
            n = n // 2
    else:
        c = int(c)
        i = b + l -1
        j = c + l
        res = 0
        while j-i > 0:
            if i & 1:
                res |= data[i]
                i += 1
            if j & 1:
                res |= data[j-1]
                j -= 1
            i //= 2
            j //= 2
        print(bin(res).count('1'))