Reformat Date

Reformat Date - Problem

String Easy

Given a date string in the form Day Month Year, convert it to the format YYYY-MM-DD.

Input Format:

Day is in the set {"1st", "2nd", "3rd", "4th", ..., "30th", "31st"}
Month is in the set {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"}
Year is in the range [1900, 2100]

Output Format:

YYYY denotes the 4-digit year
MM denotes the 2-digit month (01-12)
DD denotes the 2-digit day (01-31)

Input & Output

Example 1 — Basic Case

$ Input: date = "20th Oct 2052"

› Output: "2052-10-20"

💡 Note: Split into day=20th, month=Oct, year=2052. Remove 'th' to get day=20, Oct is month 10, format as 2052-10-20

Example 2 — Single Digit Day

$ Input: date = "6th Jun 1933"

› Output: "1933-06-06"

💡 Note: Day 6th becomes 06, Jun is month 06, year 1933. Result: 1933-06-06 with zero padding

Example 3 — End of Year

$ Input: date = "31st Dec 2099"

› Output: "2099-12-31"

💡 Note: Last day of year: 31st becomes 31, Dec is month 12, format as 2099-12-31

Constraints

Date string is always valid and in the format "Day Month Year"
Day is in the set {"1st", "2nd", "3rd", ..., "31st"}
Month is in the set {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"}
Year is in the range [1900, 2100]

Visualization

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Asked in

a Amazon 15 M Microsoft 12 f Facebook 8

The key insight is to split the date string, extract numeric values, and use a hash map for efficient month lookup. Best approach uses hash map for O(1) month conversion. Time: O(1), Space: O(1)

Common Approaches

✓ Hash Map Lookup

⏱️ Time: O(1) Space: O(1)

Create a hash map that maps month names to their numeric values. Split the input string, extract the day number, use hash map for instant month lookup, and format the result.

String Splitting with Linear Month Search

⏱️ Time: O(1) Space: O(1)

Split the input string into day, month, and year components. Remove ordinal suffixes from day, search through month names sequentially, and format the result with proper padding.

Hash Map Lookup — Algorithm Steps

Create month name to number mapping
Split input string by spaces
Extract day number from ordinal string
Look up month number in hash map
Format as YYYY-MM-DD with padding

Visualization

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Step-by-Step Walkthrough

Create Hash Map

Map {"Jan":1, "Feb":2, ..., "Dec":12}

Parse & Lookup

Split input, extract day=20, lookup Oct→10 instantly

Format Result

Combine as "2052-10-20" with proper padding

Code -

solution.c — C

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

int getMonthNumber(char* month) {
    char* months[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun",
                     "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
    for (int i = 0; i < 12; i++) {
        if (strcmp(months[i], month) == 0) {
            return i + 1;
        }
    }
    return 1;
}

char* solution(char* date) {
    char dayStr[10], monthStr[10], yearStr[10];
    sscanf(date, "%s %s %s", dayStr, monthStr, yearStr);
    
    // Extract day number (remove ordinal suffix)
    dayStr[strlen(dayStr)-2] = '\0';
    int day = atoi(dayStr);
    
    // Get month number
    int month = getMonthNumber(monthStr);
    
    int year = atoi(yearStr);
    
    char* result = (char*)malloc(11 * sizeof(char));
    sprintf(result, "%04d-%02d-%02d", year, month, day);
    return result;
}

int main() {
    char date[100];
    fgets(date, sizeof(date), stdin);
    date[strcspn(date, "\n")] = 0;
    
    char* result = solution(date);
    printf("%s\n", result);
    free(result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

Fixed operations: split (3 parts), hash map lookup is O(1), string formatting

✓ Linear Growth

Space Complexity

O(1)

Hash map stores 12 month mappings, which is constant space

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Hash Map Lookup — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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