Buggy python submission (verdict: Time Limit Exceeded):
from bisect import bisect_left, bisect, insort
def main():
    n = int(input())
    s = list(input())
    q = int(input())
    _is = {chr(i):[] for i in range(ord("a"), ord("a")+27)}
    for i, si in enumerate(s, 1):
        _is[si].append(i)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            index = bisect_left(_is[s[i-1]], i)
            _is[s[i-1]].pop(index)
            insort(_is[c], i)
            s[i-1] = c
        else:
            c = int(c)
            cnt = 0
            for _isi in _is.values():
                if _isi:
                    is_in = bisect(_isi, c)-bisect_left(_isi, i)
                    if is_in:
                        cnt += 1
            print(cnt)


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
from bisect import bisect_left, bisect, insort
def main():
    n = int(input())
    s = list(input())
    q = int(input())
    _is = {chr(i):[] for i in range(ord("a"), ord("a")+27)}
    for i, si in enumerate(s, 1):
        _is[si].append(i)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            index = bisect_left(_is[s[i-1]], i)
            del _is[s[i-1]][index]
            insort(_is[c], i)
            s[i-1] = c
        else:
            c = int(c)
            cnt = 0
            for _isi in _is.values():
                if _isi:
                    is_in = bisect(_isi, c)-bisect_left(_isi, i)
                    if is_in:
                        cnt += 1
            print(cnt)


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
from bisect import bisect_left, bisect, insort
def main():
    n = int(input())
    s = list(input())
    q = int(input())
    _is = {chr(i):[] for i in range(ord("a"), ord("a")+27)}
    for i, si in enumerate(s, 1):
        _is[si].append(i)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            _is[s[i-1]].remove(i)
            insort(_is[c], i)
            s[i-1] = c
        else:
            c = int(c)
            cnt = 0
            for _isi in _is.values():
                if _isi:
                    is_in = bisect(_isi, c)-bisect_left(_isi, i)
                    if is_in:
                        cnt += 1
            print(cnt)


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
from bisect import bisect_left, bisect, insort
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    _is = [[] for _ in range(26)]
    for i, si in enumerate(s, 1):
        _is[si].append(i)
    
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            if s[i-1] == c:
                continue
            _is[s[i-1]].remove(i)
            insort(_is[ord(c)-a], i)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for _isi in _is:
                if _isi:
                    is_in = bisect(_isi, c)-bisect_left(_isi, i)
                    if is_in:
                        cnt += 1
            print(cnt)


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
from bisect import bisect_left, bisect, insort
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    _is = [[] for _ in range(26)]
    for i, si in enumerate(s, 1):
        _is[si].append(i)
    
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            _is[s[i-1]].remove(i)
            insort(_is[ord(c)-a], i)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for j in range(26):
                is_in = bisect(_is[j], c)-bisect_left(_is[j], i)
                if is_in:
                    cnt += 1
            print(cnt)


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Runtime Error):
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            si[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()


Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    s = [None] + list(input())
    q = int(input())
    alpha = [[] for i in range(26)]
    for i, si in enumerate(s[1:], 1):
        alpha[ord(si)-97].append(i)
    check = lambda x, y, z: [i for i in z if i >= x and i <= y]
    for _ in range(q):
        i, a, b = input().split()
        a = int(a)
        if i == "1":
            alpha[ord(s[a])-97].remove(a)
            alpha[ord(b)-97].append(a)
            s[a] = b
        else:
            cnt = 0
            for i in range(26):
                if alpha[i] and check(a, int(b), alpha[i]):
                    cnt += 1
            print(cnt)

if __name__ == "__main__":
    main()

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    s = [None] + list(input())
    q = int(input())
    for _ in range(q):
        i, a, b = input().split()
        if i == "1":
            s[int(a)] = b
        else:
            print(len(set(s[int(a):int(b)+1])))

if __name__ == "__main__":
    main()

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    s = [None] + list(input())
    q = int(input())
    for _ in range(q):
        i, a, b = input().split()
        if i == "1":
            s[int(a)] = b
        else:
            print(len(set(s[int(a):int(b)+1])))

if __name__ == "__main__":
    main()

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    S = input()
    q = int(input())
    for _ in range(q):
        t, i, c = input().split()
        if t == '1':
            S = S[:int(i)-1] + c + S[int(i):] 
        else:
            print(len(set(S[int(i)-1:int(c)])))
        
if __name__ == "__main__":
    main()   

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    S = input()
    q = int(input())
    for _ in range(q):
        t, i, c = input().split()
        if t == '1':
            S = S[:int(i)-1] + c + S[int(i):] 
        else:
            print(len(set(S[int(i)-1:int(c)])))
        
if __name__ == "__main__":
    main()   

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    S = list(input())
    q = int(input())
    for _ in range(q):
        t, i, c = input().split()
        if t == '1':
            S[int(i)-1] = c
        else:
            print(len(set(S[int(i)-1:int(c)])))
        
if __name__ == "__main__":
    main()   

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Runtime Error):
def main():
    n = int(input())
    S = input()
    q = int(input())
    for _ in range(q):
        t, i, c = input().split()
        if t == '1':
            S[int(i)-1] = c
        else:
            print(len(set(S[int(i)-1:int(c)])))
        
if __name__ == "__main__":
    main()   

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    n = int(input())
    S = list(input())
    q = int(input())
    for _ in range(q):
        t, i, c = input().split()
        if t == '1':
            S[int(i)-1] = c
        else:
            print(len(set(S[int(i)-1:int(c)])))
        
if __name__ == "__main__":
    main()   

Fixed submission:
def main():
    n = int(input())
    a = ord("a")
    orda = lambda x: ord(x)-a
    s = list(map(orda, list(input())))
    q = int(input())
    bits = [BIT(n) for _ in range(26)]
    for i, si in enumerate(s, 1):
        bits[si].add(i, 1)
    for _ in range(q):
        t, i, c = input().split()
        i = int(i)
        if t == "1":
            bits[s[i-1]].add(i, -1)
            bits[ord(c)-a].add(i, 1)
            s[i-1] = ord(c)-a
        else:
            c = int(c)
            cnt = 0
            for bit in bits:
                is_in = bit.sum_(c) - bit.sum_(i-1)
                if is_in:
                    cnt += 1
            print(cnt)
            
            
class BIT:
    __slots__ = ["length", "data"]
    def __init__(self, length):
        self.data = [0]*(length+1)
        self.length = length
    
    def add(self, i, x):
        while i < self.length+1:
            self.data[i] += x
            i += i&-i
    
    def sum_(self, i):
        x = 0
        while i > 0:
            x += self.data[i]
            i -= i&-i
        return x


if __name__ == "__main__":
    main()