Buggy python submission (verdict: Time Limit Exceeded):
from collections import Counter

def main():
    n = int(input().strip())
    s = list(input().strip())

    bit = [Counter() for _ in range(n + 1)]

    def query(i):
        result = Counter()
        while i:
            result += bit[i]
            i -= i & -i
        return result
    
    def update(i, cnt):
        while i < len(bit):
            bit[i] += cnt
            i += i & -i

    for i, c in enumerate(s, 1):
        update(i, Counter({c: 1}))

    q = int(input().strip())
    for _ in range(q):
        Q = input().strip().split()
        if Q[0] == "1":
            i, c = int(Q[1]), Q[2]
            if s[i - 1] != c:
                update(i, Counter({s[i - 1]: -1, c: 1}))
                s[i - 1] = c
        else:
            cnt = query(int(Q[2])) - query(int(Q[1]) - 1)
            print(len(cnt))


main()


Fixed submission:
def main():
    n = int(input().strip())
    s = list(input().strip())

    bit = [[0] * 26 for _ in range(n + 1)]

    def query(i):
        result = [0] * 26
        while i:
            for j, x in enumerate(bit[i]):
                result[j] += x
            i -= i & -i
        return result
    
    def update(i, c, val):
        j = ord(c) - ord("a")
        while i < len(bit):
            bit[i][j] += val
            i += i & -i

    for i, c in enumerate(s, 1):
        update(i, c, 1)

    q = int(input().strip())
    for _ in range(q):
        Q = input().strip().split()
        if Q[0] == "1":
            i, c = int(Q[1]), Q[2]
            update(i, s[i - 1], -1)
            update(i, c, 1)
            s[i - 1] = c
        else:
            l, r = int(Q[1]), int(Q[2])
            cnt = [x - y for x, y in zip(query(r), query(l - 1))]
            print(sum(x > 0 for x in cnt))


main()

Buggy python submission (verdict: Time Limit Exceeded):
from collections import Counter

def main():
    n = int(input().strip())
    s = list(input().strip())

    bit = [Counter() for _ in range(n + 1)]

    def query(i):
        result = Counter()
        while i:
            result += bit[i]
            i -= i & -i
        return result
    
    def update(i, cnt):
        while i < len(bit):
            bit[i] += cnt
            i += i & -i

    for i, c in enumerate(s, 1):
        update(i, {c: 1})

    q = int(input().strip())
    for _ in range(q):
        Q = input().strip().split()
        if Q[0] == "1":
            i, c = int(Q[1]), Q[2]
            if s[i - 1] != c:
                update(i, {s[i - 1]: -1, c: 1})
                s[i - 1] = c
        else:
            cnt = query(int(Q[2])) - query(int(Q[1]) - 1)
            print(len(cnt))


main()


Fixed submission:
def main():
    n = int(input().strip())
    s = list(input().strip())

    bit = [[0] * 26 for _ in range(n + 1)]

    def query(i):
        result = [0] * 26
        while i:
            for j, x in enumerate(bit[i]):
                result[j] += x
            i -= i & -i
        return result
    
    def update(i, c, val):
        j = ord(c) - ord("a")
        while i < len(bit):
            bit[i][j] += val
            i += i & -i

    for i, c in enumerate(s, 1):
        update(i, c, 1)

    q = int(input().strip())
    for _ in range(q):
        Q = input().strip().split()
        if Q[0] == "1":
            i, c = int(Q[1]), Q[2]
            update(i, s[i - 1], -1)
            update(i, c, 1)
            s[i - 1] = c
        else:
            l, r = int(Q[1]), int(Q[2])
            cnt = [x - y for x, y in zip(query(r), query(l - 1))]
            print(sum(x > 0 for x in cnt))


main()

Buggy python submission (verdict: Runtime Error):
from collections import Counter

def main():
    n = int(input().strip())
    s = list(input().strip())

    bit = [Counter() for _ in range(n + 1)]

    def query(i):
        result = Counter()
        while i:
            result += bit[i]
            i -= i & -i
        return result
    
    def update(i, cnt):
        while i < len(bit):
            bit[i] += cnt
            i += i & -i

    for i, c in enumerate(s, 1):
        update(i, {c: 1})

    q = int(input().strip())
    for _ in range(q):
        Q = input().strip().split()
        if Q[0] == "1":
            i, c = int(Q[1]), Q[2]
            update(i, {s[i - 1]: -1, c: 1})
            s[i - 1] = c
        else:
            cnt = query(int(Q[2])) - query(int(Q[1]) - 1)
            print(len(cnt))


main()


Fixed submission:
def main():
    n = int(input().strip())
    s = list(input().strip())

    bit = [[0] * 26 for _ in range(n + 1)]

    def query(i):
        result = [0] * 26
        while i:
            for j, x in enumerate(bit[i]):
                result[j] += x
            i -= i & -i
        return result
    
    def update(i, c, val):
        j = ord(c) - ord("a")
        while i < len(bit):
            bit[i][j] += val
            i += i & -i

    for i, c in enumerate(s, 1):
        update(i, c, 1)

    q = int(input().strip())
    for _ in range(q):
        Q = input().strip().split()
        if Q[0] == "1":
            i, c = int(Q[1]), Q[2]
            update(i, s[i - 1], -1)
            update(i, c, 1)
            s[i - 1] = c
        else:
            l, r = int(Q[1]), int(Q[2])
            cnt = [x - y for x, y in zip(query(r), query(l - 1))]
            print(sum(x > 0 for x in cnt))


main()