Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python
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


# 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):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    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)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", " "), 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", "\n"))
    if kwargs.pop("flush", False):
        file.flush()


if sys.version_info[0] < 3:
    sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
else:
    sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)


from math import sqrt, floor, factorial, gcd, log
from collections import deque, Counter, defaultdict
from itertools import permutations
from bisect import bisect

input = lambda: sys.stdin.readline().rstrip("\r\n")
read = lambda: list(map(int, input().strip().split(" ")))

# sys.setrecursionlimit(10**5)



def solve():
    n = int(input()); arr = read()
    mex = 0; curr = 1
    ans = []
    last = 0
    for i in arr:
        # if i > last+2:
        #     print(-1, last, i);
        #     exit()
        # else:
        #     last = i
        if curr < i:
            print(-1);exit()
        if i == mex:
            ans.append(curr)
        else:
            ans.append(mex)
            mex = curr
            curr += 1
        

    print(*ans)






        

if __name__ == "__main__":
	solve()

Fixed submission:
import sys
from math import sqrt, gcd, ceil, log
from bisect import bisect
from collections import defaultdict
inp = sys.stdin.readline
read = lambda: list(map(int, inp().strip().split()))

# sys.setrecursionlimit(10**6)



def solve():
	s = inp().strip();
	dic = defaultdict(int); dic[0] = 1
	# sett.add(0)
	ans = 0; cum = 0
	for i in range(len(s)-1, -1, -1):
		cum += (int(s[i])*pow(10, (len(s)-i-1), 2019)) % 2019
		cum %= 2019
		dic[cum] += 1
	# print(dic)
	for i in dic:
		ans += ((dic[i])*(dic[i]-1))//2
	print(ans)






if __name__ == "__main__":
	solve()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
from math import sqrt, gcd, ceil, log
from bisect import bisect
from collections import defaultdict
inp = sys.stdin.readline
read = lambda: list(map(int, inp().strip().split()))

# sys.setrecursionlimit(10**6)



def solve():
	s = inp().strip();# dic = defaultdict(int)
	sett = set(); sett.add(0)
	ans = 0; cum = 0
	for i in range(len(s)-1, -1, -1):

		cum += (int(s[i])*(10**(len(s)-i-1)% 2019)) % 2019
		cum %= 2019
		# print((int(s[i])*(10**(len(s)-i-1)% 2019)) % 2019, i, cum)
		if cum in sett:
			ans += 1
		sett.add(cum)
	# print(sett)
	print(ans)






if __name__ == "__main__":
	solve()

Fixed submission:
import sys
from math import sqrt, gcd, ceil, log
from bisect import bisect
from collections import defaultdict
inp = sys.stdin.readline
read = lambda: list(map(int, inp().strip().split()))

# sys.setrecursionlimit(10**6)



def solve():
	s = inp().strip();
	dic = defaultdict(int); dic[0] = 1
	# sett.add(0)
	ans = 0; cum = 0
	for i in range(len(s)-1, -1, -1):
		cum += (int(s[i])*pow(10, (len(s)-i-1), 2019)) % 2019
		cum %= 2019
		dic[cum] += 1
	# print(dic)
	for i in dic:
		ans += ((dic[i])*(dic[i]-1))//2
	print(ans)






if __name__ == "__main__":
	solve()