Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    s = read()
    ans = 0
    st = []
    for ch in s:
        st.append(ch)
        if len(st) >= 3 and st[-3:] == ['A', 'B', 'C']:
            st.pop()
            st.pop()
            st.pop()
            c = 1
            while st and st[-1] == 'A':
                c += 1
                st.pop()
            ans += c
            st = list('A'*c)
    print(ans)


def slow(s):
    ans = 0
    while True:
        x = s.count('ABC')
        if x:
            ans += x
            s = s.replace('ABC', 'BCA')
        else:
            break
    return ans

# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")


def read():
    return input()


def read_array(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)


def read_int():
    return int(read())


def read_int_array(sep=None, maxsplit=-1):
    return [int(a) for a in read_array(sep, maxsplit)]

# endregion

if __name__ == "__main__":
    main()


Fixed submission:
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    s = read()
    ans = 0
    cnt_a = 0
    last_ch = 'C'
    for ch in s:
        if ch == 'A':
            if last_ch != 'A':
                cnt_a = 0
            cnt_a += 1
            last_ch = ch
        elif ch == 'B':
            if last_ch != 'A':
                cnt_a = 0
            last_ch = ch
        elif ch == 'C':
            if last_ch == 'B':
                ans += cnt_a
                cnt_a = cnt_a
                last_ch = 'A'
            else:
                cnt_a = 0
                last_ch = ch
    print(ans)


def slow(s):
    ans = 0
    while True:
        x = s.count('ABC')
        if x:
            ans += x
            s = s.replace('ABC', 'BCA')
        else:
            break
    return ans

# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")


def read():
    return input()


def read_array(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)


def read_int():
    return int(read())


def read_int_array(sep=None, maxsplit=-1):
    return [int(a) for a in read_array(sep, maxsplit)]

# endregion

if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    s = read()
    ans = 0
    st = []
    for ch in list(s):
        st.append(ch)
        if len(st) >= 3 and st[-3:] == ['A', 'B', 'C']:
            st.pop()
            st.pop()
            st.pop()
            c = 1
            while st and st[-1] == 'A':
                c += 1
                st.pop()
            st.append('B')
            st.append('C')
            ans += c
            for _ in range(c):
                st.append('A')
    print(ans)


def slow(s):
    ans = 0
    while True:
        x = s.count('ABC')
        if x:
            ans += x
            s = s.replace('ABC', 'BCA')
        else:
            break
    return ans

# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")


def read():
    return input()


def read_array(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)


def read_int():
    return int(read())


def read_int_array(sep=None, maxsplit=-1):
    return [int(a) for a in read_array(sep, maxsplit)]

# endregion

if __name__ == "__main__":
    main()


Fixed submission:
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    s = read()
    ans = 0
    cnt_a = 0
    last_ch = 'C'
    for ch in s:
        if ch == 'A':
            if last_ch != 'A':
                cnt_a = 0
            cnt_a += 1
            last_ch = ch
        elif ch == 'B':
            if last_ch != 'A':
                cnt_a = 0
            last_ch = ch
        elif ch == 'C':
            if last_ch == 'B':
                ans += cnt_a
                cnt_a = cnt_a
                last_ch = 'A'
            else:
                cnt_a = 0
                last_ch = ch
    print(ans)


def slow(s):
    ans = 0
    while True:
        x = s.count('ABC')
        if x:
            ans += x
            s = s.replace('ABC', 'BCA')
        else:
            break
    return ans

# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")


def read():
    return input()


def read_array(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)


def read_int():
    return int(read())


def read_int_array(sep=None, maxsplit=-1):
    return [int(a) for a in read_array(sep, maxsplit)]

# endregion

if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    s = read()
    ans = 0
    while True:
        x = s.count('ABC')
        if x:
            ans += x
            s = s.replace('ABC', 'BCA')
        else:
            break
    print(ans)



# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")


def read():
    return input()


def read_array(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)


def read_int():
    return int(read())


def read_int_array(sep=None, maxsplit=-1):
    return [int(a) for a in read_array(sep, maxsplit)]

# endregion

if __name__ == "__main__":
    main()


Fixed submission:
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    s = read()
    ans = 0
    cnt_a = 0
    last_ch = 'C'
    for ch in s:
        if ch == 'A':
            if last_ch != 'A':
                cnt_a = 0
            cnt_a += 1
            last_ch = ch
        elif ch == 'B':
            if last_ch != 'A':
                cnt_a = 0
            last_ch = ch
        elif ch == 'C':
            if last_ch == 'B':
                ans += cnt_a
                cnt_a = cnt_a
                last_ch = 'A'
            else:
                cnt_a = 0
                last_ch = ch
    print(ans)


def slow(s):
    ans = 0
    while True:
        x = s.count('ABC')
        if x:
            ans += x
            s = s.replace('ABC', 'BCA')
        else:
            break
    return ans

# region fastio

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        if self.buffer.tell():
            return self.buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip(b"\r\n")


def read():
    return input()


def read_array(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)


def read_int():
    return int(read())


def read_int_array(sep=None, maxsplit=-1):
    return [int(a) for a in read_array(sep, maxsplit)]

# endregion

if __name__ == "__main__":
    main()