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

n = int(input().rstrip('\n'))
s = input().rstrip('\n')
l = [c for c in s]
q = int(input().rstrip('\n'))
Q = [input().rstrip('\n').split() for _ in range(q)]

class SegTree():
    def __init__(self, n_):
        self.n = 1
        while self.n < n_:
            self.n *= 2
        self.tree = [2 for _ in range(2 * self.n - 1)]
    def update(self, k, a):
        k += self.n - 1
        self.tree[k] = a
        while k > 0:
            k = (k - 1) // 2
            self.tree[k] = min(self.tree[k * 2 + 1], self.tree[k * 2 + 2])
    def query(self, l, r):
        l += self.n - 1
        r += self.n - 2
        res = 2
        while r-l>1:
            if l&1 == 0:
                res = min(res,self.tree[l])
            if r&1 == 1:
                res = min(res,self.tree[r])
                r -= 1
            l = l//2
            r = (r-1)//2
        if l == r:
            res = min(res,self.tree[l])
        else:
            res = min(min(res,self.tree[l]),self.tree[r])
        return res

d = { c: SegTree(n) for c in "abcdefghijklmnopqrstuvwxyz"}

for i, c in enumerate(s):
    d[c].update(i, 1)


for a, b, c in Q:
    if a == "1":
        b = int(b) - 1
        d[l[b]].update(b, 2)
        d[c].update(b, 1)
        l[b] = c
    else:
        res = 0
        b, c = int(b), int(c)
        for key in d:
            if d[key].query(b-1, c) == 1:
                res += 1
        print(res)


Fixed submission:
import sys
input = sys.stdin.readline

n = int(input().rstrip('\n'))
s = input().rstrip('\n')
l = [c for c in s]
q = int(input().rstrip('\n'))
Q = [input().rstrip('\n').split() for _ in range(q)]

class SegTree():
    def __init__(self, n_):
        self.n = 1
        while self.n < n_:
            self.n *= 2
        self.tree = [2 for _ in range(2 * self.n - 1)]
    def update(self, k, a):
        k += self.n - 1
        self.tree[k] = a
        while k > 0:
            k = (k - 1) // 2
            self.tree[k] = min(self.tree[k * 2 + 1], self.tree[k * 2 + 2])
    def query(self, l, r):
        l += self.n - 1
        r += self.n - 2
        res = 2
        while r-l>1:
            if l&1 == 0:
                res = min(res,self.tree[l])
            if r&1 == 1:
                res = min(res,self.tree[r])
                r -= 1
            l = l//2
            r = (r-1)//2
        if l == r:
            res = min(res,self.tree[l])
        else:
            res = min(min(res,self.tree[l]),self.tree[r])
        return res

d = { c: SegTree(n) for c in "abcdefghijklmnopqrstuvwxyz"}

for i, c in enumerate(s):
    d[c].update(i, 1)


for a, b, c in Q:
    if a == "1":
        b = int(b) - 1
        d[l[b]].update(b, 2)
        d[c].update(b, 1)
        l[b] = c
    else:
        res = 0
        b, c = int(b), int(c)
        for key in d:
            if d[key].query(b-1, c) == 1:
                res += 1
        print(res)

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

n = int(input().rstrip('\n'))
s = input().rstrip('\n')
l = [c for c in s]
q = int(input().rstrip('\n'))
Q = [input().rstrip('\n').split() for _ in range(q)]

class SegTree():
    def __init__(self, n_):
        self.n = 1
        while self.n < n_:
            self.n *= 2
        self.tree = [float('inf') for _ in range(2 * self.n - 1)]
    def update(self, k, a):
        k += self.n - 1
        self.tree[k] = a
        while k > 0:
            k = (k - 1) // 2
            self.tree[k] = min(self.tree[k * 2 + 1], self.tree[k * 2 + 2])
    def query(self, l, r):
        l += self.n - 1
        r += self.n - 2
        res = float('inf')
        while r-l>1:
            if l&1 == 0:
                res = min(res,self.tree[l])
            if r&1 == 1:
                res = min(res,self.tree[r])
                r -= 1
            l = l//2
            r = (r-1)//2
        if l == r:
            res = min(res,self.tree[l])
        else:
            res = min(min(res,self.tree[l]),self.tree[r])
        return res

d = { c: SegTree(n) for c in "abcdefghijklmnopqrstuvwxyz"}

for i, c in enumerate(s):
    d[c].update(i, 1)


for a, b, c in Q:
    if a == "1":
        b = int(b) - 1
        # c = c.rstrip('\n')
        d[l[b]].update(b, float('inf'))
        d[c].update(b, 1)
        l[b] = c
    else:
        res = 0
        b, c = int(b), int(c)
        for key in d:
            if d[key].query(b-1, c) == 1:
                res += 1
        print(res)
# for key in d:
#     print(key)
#     print(d[key].tree[7:])


Fixed submission:
import sys
input = sys.stdin.readline

n = int(input().rstrip('\n'))
s = input().rstrip('\n')
l = [c for c in s]
q = int(input().rstrip('\n'))
Q = [input().rstrip('\n').split() for _ in range(q)]

class SegTree():
    def __init__(self, n_):
        self.n = 1
        while self.n < n_:
            self.n *= 2
        self.tree = [2 for _ in range(2 * self.n - 1)]
    def update(self, k, a):
        k += self.n - 1
        self.tree[k] = a
        while k > 0:
            k = (k - 1) // 2
            self.tree[k] = min(self.tree[k * 2 + 1], self.tree[k * 2 + 2])
    def query(self, l, r):
        l += self.n - 1
        r += self.n - 2
        res = 2
        while r-l>1:
            if l&1 == 0:
                res = min(res,self.tree[l])
            if r&1 == 1:
                res = min(res,self.tree[r])
                r -= 1
            l = l//2
            r = (r-1)//2
        if l == r:
            res = min(res,self.tree[l])
        else:
            res = min(min(res,self.tree[l]),self.tree[r])
        return res

d = { c: SegTree(n) for c in "abcdefghijklmnopqrstuvwxyz"}

for i, c in enumerate(s):
    d[c].update(i, 1)


for a, b, c in Q:
    if a == "1":
        b = int(b) - 1
        d[l[b]].update(b, 2)
        d[c].update(b, 1)
        l[b] = c
    else:
        res = 0
        b, c = int(b), int(c)
        for key in d:
            if d[key].query(b-1, c) == 1:
                res += 1
        print(res)