Buggy python submission (verdict: Time Limit Exceeded):
import sys
input = lambda: sys.stdin.readline()

N = int(input())
S = list(input().strip())
Q = int(input())

for i in range(Q):
    q, a, b = map(str, input().split())
    if int(q) == 1:
        a = int(a) - 1
        S[a] = b
    if int(q) == 2:
        a = int(a) - 1
        b = int(b) - 1
        _set = set(S[a: b+1])
        print(len(_set))


Fixed submission:
class SegmentTree:
    def __init__(self, data):
        # 単位元
        self.ide_ele = 0
        # num: n以上の最小の2のべき乗
        self.num = 2 ** (N - 1).bit_length()
        self.seg = [self.ide_ele] * 2 * self.num
        for i in range(N):
            self.seg[i + self.num - 1] = 1 << (ord(data[i]) - 97)
        for i in range(self.num - 2, -1, -1):
            self.seg[i] = self.seg[2 * i + 1] | self.seg[2 * i + 2]

    def update(self, k, x):
        k += self.num - 1
        self.seg[k] = 1 << (ord(x) - 97)
        while k:
            k = (k - 1) // 2
            self.seg[k] = self.seg[k * 2 + 1] | self.seg[k * 2 + 2]

    def query(self, left, right):
        if right <= left:
            return self.ide_ele
        left += self.num - 1
        right += self.num - 2
        res = self.ide_ele
        while right - left > 1:
            if left & 1 == 0:
                res = res | self.seg[left]
            if right & 1 == 1:
                res = res | self.seg[right]
                right -= 1
            left = left // 2
            right = (right - 1) // 2
        if left == right:
            res = res | self.seg[left]
        else:
            res = (res | self.seg[left]) | self.seg[right]
        return res


N = int(input())
S = list(input().strip())
Q = int(input())
MAX_DIGIT = 26
st = SegmentTree(S)


for i in range(Q):
    q, a, b = map(str, input().split())
    if int(q) == 1:
        a = int(a) - 1
        st.update(a, b)
    if int(q) == 2:
        a = int(a) - 1
        b = int(b)
        res = st.query(a, b)
        print(str(bin(res)).count('1'))

Buggy python submission (verdict: Time Limit Exceeded):
N = int(input())
S = list(input().strip())
Q = int(input())

for i in range(Q):
    q, a, b = map(str, input().split())
    if int(q) == 1:
        a = int(a) - 1
        S[a] = b
    if int(q) == 2:
        a = int(a) - 1
        b = int(b) - 1
        _set = set(S[a: b+1])
        print(len(_set))


Fixed submission:
class SegmentTree:
    def __init__(self, data):
        # 単位元
        self.ide_ele = 0
        # num: n以上の最小の2のべき乗
        self.num = 2 ** (N - 1).bit_length()
        self.seg = [self.ide_ele] * 2 * self.num
        for i in range(N):
            self.seg[i + self.num - 1] = 1 << (ord(data[i]) - 97)
        for i in range(self.num - 2, -1, -1):
            self.seg[i] = self.seg[2 * i + 1] | self.seg[2 * i + 2]

    def update(self, k, x):
        k += self.num - 1
        self.seg[k] = 1 << (ord(x) - 97)
        while k:
            k = (k - 1) // 2
            self.seg[k] = self.seg[k * 2 + 1] | self.seg[k * 2 + 2]

    def query(self, left, right):
        if right <= left:
            return self.ide_ele
        left += self.num - 1
        right += self.num - 2
        res = self.ide_ele
        while right - left > 1:
            if left & 1 == 0:
                res = res | self.seg[left]
            if right & 1 == 1:
                res = res | self.seg[right]
                right -= 1
            left = left // 2
            right = (right - 1) // 2
        if left == right:
            res = res | self.seg[left]
        else:
            res = (res | self.seg[left]) | self.seg[right]
        return res


N = int(input())
S = list(input().strip())
Q = int(input())
MAX_DIGIT = 26
st = SegmentTree(S)


for i in range(Q):
    q, a, b = map(str, input().split())
    if int(q) == 1:
        a = int(a) - 1
        st.update(a, b)
    if int(q) == 2:
        a = int(a) - 1
        b = int(b)
        res = st.query(a, b)
        print(str(bin(res)).count('1'))