# 5 Best Ways to Find the Latest Valid Time by Replacing Hidden Digits in Python

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π‘ Problem Formulation: You’re given a time string where some of the digits are represented by “?” and you need to find the latest valid time that can be formed by replacing these hidden digits. For example, given the input “2?:?8”, a possible output could be “23:58”. It’s important to ensure that the time constructed is valid within a 24-hour clock format.

## Method 1: Brute Force Guessing

The brute force method checks all possible combinations of numbers that could replace the “?” characters. It iterates through the time string, replacing each “?” with numbers ranging from ‘0’ to ‘9’, validating and comparing times as it goes, to find the latest possible time.

Here’s an example:

def replace_hidden_digits(time):
latest_time = "00:00"
for h1 in '123??' if time[0]=='?' else time[0]:
for h2 in '0123456789' if time[1]=='?' else time[1]:
for m1 in '012345?' if time[3]=='?' else time[3]:
for m2 in '0123456789' if time[4]=='?' else time[4]:
candidate = f"{h1}{h2}:{m1}{m2}"
if "??:" not in candidate and "23:5?" >= candidate >= "00:00":
latest_time = max(latest_time, candidate)
return latest_time

print(replace_hidden_digits("2?:?8"))

Output:

"23:58"

This code snippet defines a function replace_hidden_digits(time) that takes a time string with hidden digits represented as “?”. It tries all possible combinations for each unknown digit and keeps track of the latest valid time. The result is governed by the 24-hour time format constraints.

## Method 2: Smart Conditional Replacements

This method follows a more intelligent approach, minimizing the tryouts by respecting the constraints of a clock – for instance, if the first digit is a ‘2’, the second digit can only be ‘0’, ‘1’, ‘2’, or ‘3’. It replaces ‘?’ based on its position and the valid range of digits for that position.

Here’s an example:

def find_latest_time(time):
time = list(time)
if time[0] == '?':
time[0] = '2' if time[1] in '0123?' else '1'
if time[1] == '?':
time[1] = '3' if time[0] == '2' else '9'
if time[3] == '?':
time[3] = '5'
if time[4] == '?':
time[4] = '9'
return "".join(time)

print(find_latest_time("2?:?8"))

Output:

"23:58"

The function find_latest_time(time) intelligently replaces hidden digits in a given string, considering the rules of the 24-hour format. It’s a more efficient approach compared to brute force, as it limits the number of replacements by using time constraints.

## Method 3: Regular Expressions

With regular expressions, we can match patterns to find if a character is a hidden digit and then replace it with the maximum possible value allowed for that position in the time string.

Here’s an example:

import re

def latest_time_regex(time):
time = re.sub(r'(?<=^2.)\?|(?<=^.)\?', lambda x: '3' if time[0] == '2' else '9', time[:2]) + time[2:]
time = re.sub(r'\?', '5', time[:3]) + re.sub(r'\?', '9', time[3:])
return time

print(latest_time_regex("2?:?8"))

Output:

"23:58"

By employing re.sub(), the function latest_time_regex(time) uses regular expressions to replace hidden digits in the time format string. Look-behind assertions help to identify the correct replacement based on the adjacent characters.

## Method 4: Using the datetime Library

The datetime library can be utilized to parse, modify, and output time strings. We can iterate only through the likely candidates, then parse and compare the times using datetime operations.

Here’s an example:

from datetime import datetime

def latest_valid_time(time):
best_time = datetime.min.time()
for h1 in [time[0], '2', '1', '0'] if time[0] == '?' else [time[0]]:
for h2 in [time[1], '3', '0'] if time[1] == '?' else [time[1]]:
for m1 in [time[3], '5', '0'] if time[3] == '?' else [time[3]]:
for m2 in [time[4], '9', '0'] if time[4] == '?' else [time[4]]:
try:
current_time = datetime.strptime(f"{h1}{h2}:{m1}{m2}", "%H:%M").time()
if current_time > best_time:
best_time = current_time
except ValueError:
pass
return best_time.strftime("%H:%M")

print(latest_valid_time("2?:?8"))

Output:

"23:58"

The function latest_valid_time(time) uses the datetime library to parse and find the latest valid time. It iterates through possible replacement candidates only and then formats the best one to a string representing the time.

## Bonus One-Liner Method 5: The Power of itertools

By combing the itertools product function with conditional logic, we can concisely generate all valid combinations for hours and minutes and pick the latest one.

Here’s an example:

from itertools import product

def latest_time_itertools(time):
hours = [''.join(h) for h in product(*[(c if c != '?' else '012') for c in time[:2]])  if h[0] != '2' or h[1] in '0123']
minutes = [''.join(m) for m in product(*[(c if c != '?' else '0123456789') for c in time[3:]])]
return max([f"{h}:{m}" for h in hours for m in minutes if int(h) < 24 and int(m) < 60])

print(latest_time_itertools("2?:?8"))

Output:

"23:58"

latest_time_itertools(time) uses Python’s itertools.product to generate permutations of possible digits, which it then filters to maintain the valid hour and minute combinations, ultimately returning the latest time.

## Summary/Discussion

• Method 1: Brute Force Guessing. Strengths: Straightforward and simple. Weaknesses: Can be inefficient with a high number of unknown digits.
• Method 2: Smart Conditional Replacements. Strengths: More efficient by utilizing the constraints of time. Weaknesses: Less general, and the logic has to be updated for changes in format.
• Method 3: Regular Expressions. Strengths: Compact and powerful for pattern matching. Weaknesses: Can be less readable and harder to debug.
• Method 4: Using the datetime Library. Strengths: Ensures valid time and leverages well-tested library functions. Weaknesses: Potentially overkill for simple time manipulations and not as efficient as other methods.
• Method 5: The Power of itertools. Strengths: Concise and Pythonic. Weaknesses: It may generate many unnecessary combinations before applying filters.