Buggy python submission (verdict: Time Limit Exceeded):
class SegmentTree:
    def __init__(self, a):      
        self.n = len(a)
        self.N = 2 ** (self.n-1).bit_length()
        l = []
        for _a in a:
            l.append({'dic':{_a:1}, 'set':set(_a)})
        self.data = [{'dic':{}, 'set':set()} for i in range(self.N-1)] + l + [{'dic':{}, 'set':set()} for i in range(self.N-self.n)]
        for i in range(2*self.N-2, 0, -2):
            self.data[(i-1)//2] = self.init_func(self.data[i], self.data[i-1])
    
    def init_func(self, data1, data2):
        s = data1['set'].copy()
        d = data1['dic'].copy()
        for k in data2['dic']:
            if k in d:
                d[k] += data2['dic'][k]
            else:
                d[k] = data2['dic'][k]
                s.add(k)
        return {'dic':d, 'set':s}
    
    def func(self, idx, x1, x2):
        self.data[idx]['dic'][x1] -= 1
        self.data[idx]['set'].add(x2)
        if self.data[idx]['dic'][x1] == 0:
            self.data[idx]['set'].remove(x1)
        if x2 in self.data[idx]['dic']:
            self.data[idx]['dic'][x2] += 1
        else:
            self.data[idx]['dic'][x2] = 1
    
    def update(self, i, x):
        idx = self.N - 1 + i
        pre = list(self.data[idx]['dic'].keys())[0]
        self.data[idx] = {'dic':{x:1}, 'set':set(x)}
        while idx:
            idx = (idx-1) // 2
            self.func(idx, pre, x)
    
    def query(self, i, j):
        # [i, j)
        if i == j:
            return len(self.data[self.N - 1 + i]['set'])
        else:
            idx1 = self.N - 1 + i
            idx2 = self.N - 2 + j # 閉区間にする
            result = set()
            while idx1 + 1 < idx2:
                if idx1&1 == 0: # idx1が偶数
                    result = result | self.data[idx1]['set']
                if idx2&1 == 1: # idx2が奇数
                    result = result | self.data[idx2]['set']
                    idx2 -= 1
                
                idx1 //= 2
                idx2 = (idx2 - 1)//2
            
            if idx1 == idx2:
                result = result | self.data[idx1]['set']
            else: # idx1 + 1 == idx2
                result = result | self.data[idx1]['set'] | self.data[idx2]['set']
            
            return len(result)

N = int(input())
S = list(input())
st = SegmentTree(S)

Q = int(input())
for i in range(Q):
  t, a, b = input().split()
  if t == '1':
    a = int(a)
    if S[a-1] == b:
      continue
    st.update(a-1, b)
    S[a-1] = b
  else:
    a, b = int(a), int(b)
    print(st.query(a-1, b))

Fixed submission:
class SegmentTree:
    def __init__(self, a):    
        self.n = len(a)
        self.N = 2 ** (self.n-1).bit_length()
        l = []
        for _a in a:
            l.append(1 << ord(_a)-97)
        self.data = [0]*(self.N-1) + l + [0]*(self.N-self.n)
        for i in range(2*self.N-2, 0, -2):
            self.data[(i-1)//2] = self.data[i] | self.data[i-1]
    
    def update(self, i, x):
        idx = self.N - 1 + i
        self.data[idx] = 1 << ord(x)-97
        while idx:
            idx = (idx-1) // 2
            self.data[idx] = self.data[2*idx+1] | self.data[2*idx+2]
    
    def query(self, i, j):
        # [i, j)
        if i == j:
            return bin(self.data[self.N - 1 + i]).count('1')
        else:
            idx1 = self.N - 1 + i
            idx2 = self.N - 2 + j # 閉区間にする
            b = 0
            while idx1 + 1 < idx2:
                if idx1&1 == 0: # idx1が偶数
                    b = b | self.data[idx1]
                if idx2&1 == 1: # idx2が奇数
                    b = b | self.data[idx2]
                    idx2 -= 1
                
                idx1 //= 2
                idx2 = (idx2 - 1)//2
            
            if idx1 == idx2:
                b = b | self.data[idx1]
            else: # idx1 + 1 == idx2
                b = b | self.data[idx1] | self.data[idx2]
            
            return bin(b).count('1')

N = int(input())
S = list(input())
st = SegmentTree(S)

Q = int(input())
for i in range(Q):
  t, a, b = input().split()
  if t == '1':
    a = int(a)
    if S[a-1] == b:
      continue
    st.update(a-1, b)
    S[a-1] = b
  else:
    a, b = int(a), int(b)
    print(st.query(a-1, b))