Buggy python submission (verdict: Time Limit Exceeded):
n=int(input())
s=input()
q=int(input())
dic=dict()
for i,j in enumerate("qwertyuioplkjhgfdsazxcvbnm"):
    dic[j]=i
No=2
while n>No:
    No*=2
tree=[set()]*(2*No)
def add(k,x):
    k+=No-1
    tree[k]=set([dic[x]])
    while k>0:
        k=(k-1)//2
        tree[k]=tree[2*k+1]|tree[2*k+2]
def query(l,r):
    L=l+No-1
    R=r+No-1
    s=set()
    while L<=R:
        if R&1:
            s|=tree[R]
            R-=2
        else:
            R-=1
        if L&1:
            L-=1
        else:
            s|=tree[L]
        L>>=1;R>>=1
    return len(s)
for i,j in enumerate(s):
    add(i,j)
for i in range(q):
    a,b,c=input().split()
    a,b=int(a),int(b)
    if a==1:
        add(b-1,c)
    if a==2:
        c=int(c)
        print(query(b-1,c-1))

Fixed submission:
n=int(input())
s=input()
q=int(input())
dic=dict()
for i,j in enumerate("qwertyuioplkjhgfdsazxcvbnm"):
    dic[j]=i
No=2
while n>No:
    No*=2
tree=[set()]*(2*No)
def add(k,x):
    k+=No-1
    tree[k]=set([dic[x]])
    while k>0:
        k=(k-1)//2
        tree[k]=tree[2*k+1]|tree[2*k+2]
def query(l,r):
    L=l+No-1
    R=r+No-1
    s=set()
    while L<=R:
        if R&1:
            s|=tree[R]
            R-=2
        else:
            R-=1
        if L&1:
            L-=1
        else:
            s|=tree[L]
        L>>=1;R>>=1
    return len(s)
for i,j in enumerate(s):
    tree[i+No-1]=set([dic[j]])
for i in range(No-2,-1,-1):
    tree[i]=tree[2*i+1]|tree[2*i+2]
for i in range(q):
    a,b,c=input().split()
    a,b=int(a),int(b)
    if a==1:
        add(b-1,c)
    if a==2:
        c=int(c)
        print(query(b-1,c-1))

Buggy python submission (verdict: Time Limit Exceeded):
n=int(input())
s=input()
q=int(input())
No=2
while n>No:
    No*=2
tree=[set()]*(2*No)
def add(k,x):
    k+=No-1
    tree[k]=set([x])
    while k>0:
        k=(k-1)//2
        tree[k]=tree[2*k+1]|tree[2*k+2]
def query(l,r):
    L=l+No-1
    R=r+No-1
    s=set()
    while L<=R:
        if R&1:
            s|=tree[R]
            R-=2
        else:
            R-=1
        if L&1:
            L-=1
        else:
            s|=tree[L]
        L>>=1;R>>=1
    return len(s)
for i,j in enumerate(s):
    add(i,j)
for i in range(q):
    a,b,c=input().split()
    a,b=int(a),int(b)
    if a==1:
        add(b-1,c)
    if a==2:
        c=int(c)
        print(query(b-1,c-1))

Fixed submission:
n=int(input())
s=input()
q=int(input())
dic=dict()
for i,j in enumerate("qwertyuioplkjhgfdsazxcvbnm"):
    dic[j]=i
No=2
while n>No:
    No*=2
tree=[set()]*(2*No)
def add(k,x):
    k+=No-1
    tree[k]=set([dic[x]])
    while k>0:
        k=(k-1)//2
        tree[k]=tree[2*k+1]|tree[2*k+2]
def query(l,r):
    L=l+No-1
    R=r+No-1
    s=set()
    while L<=R:
        if R&1:
            s|=tree[R]
            R-=2
        else:
            R-=1
        if L&1:
            L-=1
        else:
            s|=tree[L]
        L>>=1;R>>=1
    return len(s)
for i,j in enumerate(s):
    tree[i+No-1]=set([dic[j]])
for i in range(No-2,-1,-1):
    tree[i]=tree[2*i+1]|tree[2*i+2]
for i in range(q):
    a,b,c=input().split()
    a,b=int(a),int(b)
    if a==1:
        add(b-1,c)
    if a==2:
        c=int(c)
        print(query(b-1,c-1))

Buggy python submission (verdict: Time Limit Exceeded):
n=int(input())
s=input()
q=int(input())
No=2
while n>No:
    No*=2
tree=[set()]*(2*No)
def add(k,x):
    k+=No-1
    tree[k]=set([x])
    while k>0:
        k=(k-1)//2
        tree[k]=tree[2*k+1]|tree[2*k+2]
def query(l,r):
    L=l+No-1
    R=r+No-1
    s=set()
    while L<=R:
        if R&1:
            s|=tree[R]
            R-=2
        else:
            R-=1
        if L&1:
            L-=1
        else:
            s|=tree[L]
        L>>=1;R>>=1
    return len(s)
for i,j in enumerate(s):
    add(i,j)
for i in range(q):
    a,b,c=input().split()
    a,b=int(a),int(b)
    if a==1:
        add(b-1,c)
    if a==2:
        c=int(c)
        print(query(b-1,c-1))

Fixed submission:
n=int(input())
s=input()
q=int(input())
dic=dict()
for i,j in enumerate("qwertyuioplkjhgfdsazxcvbnm"):
    dic[j]=i
No=2
while n>No:
    No*=2
tree=[set()]*(2*No)
def add(k,x):
    k+=No-1
    tree[k]=set([dic[x]])
    while k>0:
        k=(k-1)//2
        tree[k]=tree[2*k+1]|tree[2*k+2]
def query(l,r):
    L=l+No-1
    R=r+No-1
    s=set()
    while L<=R:
        if R&1:
            s|=tree[R]
            R-=2
        else:
            R-=1
        if L&1:
            L-=1
        else:
            s|=tree[L]
        L>>=1;R>>=1
    return len(s)
for i,j in enumerate(s):
    tree[i+No-1]=set([dic[j]])
for i in range(No-2,-1,-1):
    tree[i]=tree[2*i+1]|tree[2*i+2]
for i in range(q):
    a,b,c=input().split()
    a,b=int(a),int(b)
    if a==1:
        add(b-1,c)
    if a==2:
        c=int(c)
        print(query(b-1,c-1))