Latest Time by Replacing Hidden Digits - Practice Coding Problems

Latest Time by Replacing Hidden Digits - Problem

String Greedy Easy

You are given a string time in the form of hh:mm, where some of the digits in the string are hidden (represented by ?).

The valid times are those inclusively between 00:00 and 23:59.

Return the latest valid time you can get from time by replacing the hidden digits.

Input & Output

Example 1 — Missing Hour and Minute

$ Input: time = "2?:?0"

› Output: "23:50"

💡 Note: First hour digit is given as 2, so second can be max 3 (giving 23). First minute digit can be max 5, second is given as 0.

Example 2 — Missing All Digits

$ Input: time = "??:??"

› Output: "23:59"

💡 Note: All digits are missing, so we choose maximum for each: 2, 3, 5, 9 giving 23:59.

Example 3 — First Hour Digit Forces Constraint

$ Input: time = "0?:??"

› Output: "09:59"

💡 Note: First hour digit is 0, so second can be max 9. Minutes can be max 59.

Constraints

time is in the format hh:mm
It is guaranteed that you can generate a valid time from the given string

Visualization

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Understanding the Visualization

Input

Time string with ? for missing digits

Process

Replace each ? with maximum valid digit

Output

Latest possible valid time

Key Takeaway

🎯 Key Insight: Greedily choose the maximum valid digit at each position based on 24-hour time constraints

Asked in

G Google 15 a Amazon 12 M Microsoft 8

The key insight is to greedily choose the maximum valid digit for each position. The optimal greedy approach works left to right, considering time format constraints. Time: O(1), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Brute Force - Try All Times	O(1)	O(1)	Generate all possible times from 23:59 down to 00:00 and find the first match
Greedy - Choose Maximum Valid Digits	O(1)	O(1)	For each position, choose the largest valid digit based on time constraints

Brute Force - Try All Times — Algorithm Steps

Step 1: Iterate from 23:59 down to 00:00
Step 2: For each time, check if it matches the pattern
Step 3: Return first matching time

Visualization

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Step-by-Step Walkthrough

Start High

Begin checking from 23:59

Match Check

See if time matches pattern

Found

Return first match

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <stdbool.h>

bool matchesPattern(const char* t, const char* pattern) {
    for (int i = 0; i < strlen(t); i++) {
        if (pattern[i] != '?' && pattern[i] != t[i]) {
            return false;
        }
    }
    return true;
}

char* solution(char* time) {
    static char result[6];
    // Try all times from 23:59 down to 00:00
    for (int h = 23; h >= 0; h--) {
        for (int m = 59; m >= 0; m--) {
            sprintf(result, "%02d:%02d", h, m);
            if (matchesPattern(result, time)) {
                return result;
            }
        }
    }
    strcpy(result, "00:00");
    return result;
}

int main() {
    char time[6];
    fgets(time, sizeof(time), stdin);
    time[strcspn(time, "\n")] = 0; // Remove newline
    char* result = solution(time);
    printf("%s\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

At most 1440 iterations (24*60), which is constant

✓ Linear Growth

Space Complexity

O(1)

Only uses a few variables for iteration

✓ Linear Space

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Medium Frequency

~8 min Avg. Time

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Smart Actions

💡 Explanation

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Brute Force - Try All Times — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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