Buggy python submission (verdict: Runtime Error):
S = input()

def solve(S):
    mod = 10**9+7
    N = len(S)
    Aquest = []
    Cquest = []
    cntA,cntquest = 0,0
    for i in range(N):
        if S[i] in 'B?':
            Aquest.append([cntA,cntquest])
        if S[i]=='A':
            cntA += 1
        if S[i]=='?':
            cntquest += 1
    cntC,cntquest = 0,0
    for i in range(N-1,-1,-1):
        if S[i] in 'B?':
            Cquest.append([cntC,cntquest])
        if S[i]=='C':
            cntC += 1
        if S[i]=='?':
            cntquest += 1
    Cquest = Cquest[::-1]
    ans = 0
    for aquest,cquest in zip(Aquest,Cquest):
        a = aquest[0]*pow(3,aquest[1],mod)+aquest[1]*pow(3,aquest[1]-1,mod)
        c = cquest[0]*pow(3,cquest[1],mod)+cquest[1]*pow(3,cquest[1]-1,mod)
        ans += a*c%mod
    ans %= mod
    return ans
print(solve(S))

Fixed submission:
S = input()

def solve(S):
    mod = 10**9+7
    N = len(S)
    Aquest = []
    Cquest = []
    cntA,cntquest = 0,0
    for i in range(N):
        if S[i] in 'B?':
            Aquest.append([cntA,cntquest])
        if S[i]=='A':
            cntA += 1
        if S[i]=='?':
            cntquest += 1
    cntC,cntquest = 0,0
    for i in range(N-1,-1,-1):
        if S[i] in 'B?':
            Cquest.append([cntC,cntquest])
        if S[i]=='C':
            cntC += 1
        if S[i]=='?':
            cntquest += 1
    Cquest = Cquest[::-1]
    ans = 0
    for aquest,cquest in zip(Aquest,Cquest):
        a = aquest[0]*pow(3,aquest[1],mod)
        if aquest[1]>0:
            a += aquest[1]*pow(3,aquest[1]-1,mod)
        c = cquest[0]*pow(3,cquest[1],mod)
        if cquest[1]>0:
            c += cquest[1]*pow(3,cquest[1]-1,mod)
        ans += a*c%mod
    ans %= mod
    return ans
print(solve(S))

Buggy python submission (verdict: Runtime Error):
S = input()

def solve(S):
    mod = 10**9+7
    N = len(S)
    Aquest = []
    Cquest = []
    cntA,cntquest = 0,0
    for i in range(N):
        if S[i] in 'B?':
            Aquest.append([cntA,cntquest])
        if S[i]=='A':
            cntA += 1
        if S[i]=='?':
            cntquest += 1
    cntC,cntquest = 0,0
    for i in range(N-1,-1,-1):
        if S[i] in 'B?':
            Cquest.append([cntC,cntquest])
        if S[i]=='C':
            cntC += 1
        if S[i]=='?':
            cntquest += 1
    Cquest = Cquest[::-1]
    ans = 0
    for aquest,cquest in zip(Aquest,Cquest):
        a = aquest[0]*pow(3,aquest[1],mod)+aquest[1]*pow(3,aquest[1]-1,mod)
        c = cquest[0]*pow(3,cquest[1],mod)+cquest[1]*pow(3,cquest[1]-1,mod)
        ans += a*c
        ans %= mod
    return ans
print(solve(S))

Fixed submission:
S = input()

def solve(S):
    mod = 10**9+7
    N = len(S)
    Aquest = []
    Cquest = []
    cntA,cntquest = 0,0
    for i in range(N):
        if S[i] in 'B?':
            Aquest.append([cntA,cntquest])
        if S[i]=='A':
            cntA += 1
        if S[i]=='?':
            cntquest += 1
    cntC,cntquest = 0,0
    for i in range(N-1,-1,-1):
        if S[i] in 'B?':
            Cquest.append([cntC,cntquest])
        if S[i]=='C':
            cntC += 1
        if S[i]=='?':
            cntquest += 1
    Cquest = Cquest[::-1]
    ans = 0
    for aquest,cquest in zip(Aquest,Cquest):
        a = aquest[0]*pow(3,aquest[1],mod)
        if aquest[1]>0:
            a += aquest[1]*pow(3,aquest[1]-1,mod)
        c = cquest[0]*pow(3,cquest[1],mod)
        if cquest[1]>0:
            c += cquest[1]*pow(3,cquest[1]-1,mod)
        ans += a*c%mod
    ans %= mod
    return ans
print(solve(S))

Buggy python submission (verdict: Runtime Error):
S = input()

def solve(S):
    mod = 10**9+7
    N = len(S)
    Aquest = []
    Cquest = []
    cntA,cntquest = 0,0
    for i in range(N):
        if S[i] in 'B?':
            Aquest.append([cntA,cntquest])
        if S[i]=='A':
            cntA += 1
        if S[i]=='?':
            cntquest += 1
    cntC,cntquest = 0,0
    for i in range(N-1,-1,-1):
        if S[i] in 'B?':
            Cquest.append([cntC,cntquest])
        if S[i]=='C':
            cntC += 1
        if S[i]=='?':
            cntquest += 1
    Cquest = Cquest[::-1]
    ans = 0
    for aquest,cquest in zip(Aquest,Cquest):
        a = aquest[0]*pow(3,aquest[1],mod)+aquest[1]*pow(3,aquest[1]-1,mod)
        c = cquest[0]*pow(3,cquest[1],mod)+cquest[1]*pow(3,cquest[1]-1,mod)
        ans += a*c
        ans %= mod
    return ans
print(solve(S))

Fixed submission:
S = input()

def solve(S):
    mod = 10**9+7
    N = len(S)
    Aquest = []
    Cquest = []
    cntA,cntquest = 0,0
    for i in range(N):
        if S[i] in 'B?':
            Aquest.append([cntA,cntquest])
        if S[i]=='A':
            cntA += 1
        if S[i]=='?':
            cntquest += 1
    cntC,cntquest = 0,0
    for i in range(N-1,-1,-1):
        if S[i] in 'B?':
            Cquest.append([cntC,cntquest])
        if S[i]=='C':
            cntC += 1
        if S[i]=='?':
            cntquest += 1
    Cquest = Cquest[::-1]
    ans = 0
    for aquest,cquest in zip(Aquest,Cquest):
        a = aquest[0]*pow(3,aquest[1],mod)
        if aquest[1]>0:
            a += aquest[1]*pow(3,aquest[1]-1,mod)
        c = cquest[0]*pow(3,cquest[1],mod)
        if cquest[1]>0:
            c += cquest[1]*pow(3,cquest[1]-1,mod)
        ans += a*c%mod
    ans %= mod
    return ans
print(solve(S))