What is the time complexity of Reformat Date?

The optimal solution has a time complexity of O(1).

What data structures are used to solve Reformat Date?

This problem uses String. Understanding these concepts is essential for solving this Easy problem efficiently.

What are the key insights for solving Reformat Date?

The input date string follows a consistent format, allowing for straightforward string manipulation techniques (splitting, substring extraction) to separate the day, month, and year components. A mapping (e.g., a dictionary/hash map/switch statement) is needed to translate the three-letter month abbreviations to their corresponding two-digit numerical representations.

Which companies ask LeetCode 1507. Reformat Date in interviews?

This problem is frequently asked at Celigo, Expedia, Twilio, Veritas.

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Reformat Date - LeetCode 1507 Solution

Reformat Date - Complete Solution Guide

Reformat Date is LeetCode problem 1507, a Easy level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

Given a date string in the form Day Month Year , where: 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] . Convert the date string to the format YYYY-MM-DD , where: YYYY denotes the 4 digit year. MM denotes the 2 digit month. DD denotes the 2 digit day. Example 1: Input: date = "20th Oct 2052" Output: "2052-10-20" Example 2: Input: date

Detailed Explanation

The problem requires reformatting a date string from a given format ("Day Month Year") to the standard ISO 8601 format ("YYYY-MM-DD"). The input date string adheres to a specific pattern: the day is represented with an ordinal suffix (e.g., "1st", "2nd", "3rd"), the month is a three-letter abbreviation (e.g., "Jan", "Feb", "Mar"), and the year is a four-digit number. The task involves extracting these components, converting them to the desired format, and concatenating them to produce the output string.

Solution Approach

All provided solutions follow a similar approach. They first split the input string into its day, month, and year components. Then, they use a lookup mechanism (dictionary, hashmap, or switch statement) to convert the month abbreviation to its numerical equivalent. Finally, they format the day and month to two digits using zero-padding and concatenate the year, month, and day to produce the output string in "YYYY-MM-DD" format.

Step-by-Step Algorithm

Split the input date string into three parts: day, month, and year.
Extract the numerical day value from the day part by removing the ordinal suffix (e.g., remove "th", "st", "nd", "rd").
Convert the three-letter month abbreviation to its two-digit numerical representation using a pre-defined mapping (dictionary, hashmap, or switch statement).
Zero-pad the day and month values to ensure they have two digits each.
Concatenate the year, month, and day parts with hyphens to form the final "YYYY-MM-DD" formatted date string.
Return the formatted date string.

Key Insights

The input date string follows a consistent format, allowing for straightforward string manipulation techniques (splitting, substring extraction) to separate the day, month, and year components.
A mapping (e.g., a dictionary/hash map/switch statement) is needed to translate the three-letter month abbreviations to their corresponding two-digit numerical representations.
Zero-padding is necessary for both the day and month components to ensure they are always represented with two digits (e.g., "01" instead of "1").

Complexity Analysis

Time Complexity: O(1)

Space Complexity: O(1)

Topics

This problem involves: String.

Companies

Asked at: Celigo, Expedia, Twilio, Veritas.

Reformat Date | LeetCode 1507

Easy

String

Time: O(1)Space: O(1)

Solution Code

Available in:

class Solution {
    public String reformatDate(String date) {
        String[] parts = date.split(" ");
        String day = parts[0].replaceAll("[^0-9]", "");
        String month = "";
        switch (parts[1]) {
            case "Jan":
                month = "01";
                break;
            case "Feb":
                month = "02";
                break;
            case "Mar":
                month = "03";
                break;
            case "Apr":
                month = "04";
                break;
            case "May":
                month = "05";
                break;
            case "Jun":
                month = "06";
                break;
            case "Jul":
                month = "07";
                break;
            case "Aug":
                month = "08";
                break;
            case "Sep":
                month = "09";
                break;
            case "Oct":
                month = "10";
                break;
            case "Nov":
                month = "11";
                break;
            case "Dec":
                month = "12";
                break;
        }
        String year = parts[2];
        return year + "-" + month + "-" + (day.length() == 1 ? "0" + day : day);
    }
}

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Problem Info

Description

Problem 1507: Reformat Date - Detailed explanation and solution approaches.

Topics

String

Companies

CeligoExpediaTwilioVeritas

Stats

Difficulty:Easy

Languages:4

Companies:4

Problem #:1507

Problem Explanation

💡

Think Before You Code

🤔 Ask Yourself

→What is the input? What output do I need?
→Can I solve a smaller version of this problem first?
→What patterns have I seen in similar problems?

✏️ Try This First

Before looking at the solution, try to solve it on paper with a simple example!

🚀 Try on LeetCode

Solution Approach

📋

Step-by-Step Solution

6 steps

1
Split the input date string into three parts: day, month, and year.
2
Extract the numerical day value from the day part by removing the ordinal suffix (e.g., remove "th", "st", "nd", "rd").
3
Convert the three-letter month abbreviation to its two-digit numerical representation using a pre-defined mapping (dictionary, hashmap, or switch statement).
4
Zero-pad the day and month values to ensure they have two digits each.
5
Concatenate the year, month, and day parts with hyphens to form the final "YYYY-MM-DD" formatted date string.
6
Return the formatted date string.

💡

Pro Tip for Students

Try tracing through each step with a small example on paper before coding. Understanding "why" each step happens is more important than memorizing the code!

⚡

Complexity Analysis

⏱️

Time Complexity

O(1)

⚡ Constant time - Super fast! Same speed regardless of input size.

💾

Space Complexity

O(1)

💪 Constant space - Uses same memory regardless of input!

📝

Why?

The number of operations performed is constant, regardless of the input date string's length. The splitting, substring extraction, and lookup operations take constant time, and the string concatenation operation also takes constant time because the lengths of the strings involved are fixed. The space used is also constant because only a fixed number of variables are used to store the day, month, year, and intermediate results.

📖New to Big O? Click to learn more

Big O Notation - Quick Guide

O(1)Best - Instant

O(log n)Excellent - Binary Search

O(n)Good - One loop

O(n log n)OK - Sorting

O(n²)Slow - Nested loops

O(2ⁿ)Very Slow - Recursion

Alternative Approaches

Regular Expressions

Use regular expressions to extract the day, month, and year components from the input string. Then use a similar approach as above to format and combine them.

Time: O(1)Space: O(1)

Trade-offs: Regular expressions can be more concise but might be less readable for beginners. The performance difference is negligible in this case.

✨

Key Insights

1The input date string follows a consistent format, allowing for straightforward string manipulation techniques (splitting, substring extraction) to separate the day, month, and year components.
2A mapping (e.g., a dictionary/hash map/switch statement) is needed to translate the three-letter month abbreviations to their corresponding two-digit numerical representations.
3Zero-padding is necessary for both the day and month components to ensure they are always represented with two digits (e.g., "01" instead of "1").

📚

Key Terms Explained

For Students

String

A sequence of characters. Like a word or sentence in programming!

Edge Cases

Single-digit days: The solution should handle single-digit days (e.g., "1st") by zero-padding them to "01". All provided solutions address this correctly.
Month abbreviation lookup: The solutions need to handle all possible month abbreviations correctly. If a month abbreviation is not in the mapping, it will result in an error. However, the problem statement guarantees valid input.

⚠️

Common Mistakes to Avoid

✗Incorrectly handling ordinal suffixes: Forgetting to remove the ordinal suffixes ("st", "nd", "rd", "th") from the day part before converting it to an integer.

✗Not handling single-digit days and months: Failing to zero-pad single-digit days and months to ensure the "YYYY-MM-DD" format is always followed.

✅

Learning Tip

Every mistake is a learning opportunity! When you make an error, understand why it happened before moving on.

🌍

Where is This Used in Real Life?

💼Data processing and cleaning: Reformatting dates is a common task in data processing pipelines. Applications might include converting dates from different formats into a unified standard for easier analysis and reporting.

🏦Database management: Many database systems require dates to be in a specific format. This algorithm could be used as part of a data import or update process.

💡Understanding real-world applications helps you see the "why" behind algorithms. It's not just about passing interviews—these concepts power the apps and websites you use every day!

Author's Notes

💡 Key Insight

String problems often involve sliding windows or two pointers. Consider if the character set is limited (lowercase letters = only 26 possibilities).

🎯 Interview Tip

Use this problem to demonstrate clean code practices. Variable naming and code organization matter.

⚠️ Watch Out

Mutating strings directly in languages where strings are immutable. Use StringBuilder or arrays.

Written by Saswata Mondal

Software Engineer • Algorithms Enthusiast • Interview Coach

👨‍💻

About the Author

👨‍💻

Saswata MondalAuthor400+ LC Problems

B.Tech CSE, IEM Kolkata | LeetCode Rating: 1672 (Top 15%)

Certified: IBM Java Developer, NPTEL Programming in Java

Last updated: August 7, 2024LeetCode Profile GitHub

This solution has been crafted based on my experience solving 400+ LeetCode problems. Each explanation is designed to help you understand the underlying concepts, not just memorize the code.

Community Tips & Insights

I was asked this exact problem at my Google interview. The interviewer wanted me to optimize space complexity.

@InterviewProJul 13

Try solving this without using extra space first. It's a great exercise for understanding the underlying pattern.

@CodeNinjaJul 6

In a real interview, always clarify constraints first. Ask about input size and value ranges.

@TechLeadJun 29

❓

Frequently Asked Questions

What is the sliding window technique for string problems?

Sliding window maintains a "window" of characters with two pointers. Expand the right pointer to include characters, shrink from left when window violates constraints. Useful for substring problems, anagrams, and longest/shortest substring with properties.

What should I learn from easy problems?

Easy problems introduce core patterns that appear in harder problems. Master basic operations (iteration, conditionals, simple data structures), recognize common patterns (counting, searching, basic transformations), and practice explaining your thought process clearly.

What is the importance of time and space complexity analysis?

Complexity analysis is crucial because: 1) Interviewers always ask about it, 2) It helps you choose between approaches, 3) It demonstrates CS fundamentals. Always state both time and space complexity, and be prepared to explain how you derived them.

What is the optimal approach for "Reformat Date"?

For this easy-level String problem, the key is to identify the core pattern. Analyze the input constraints to determine acceptable time complexity. Consider whether standard techniques can optimize the solution. Always handle edge cases and validate your approach with examples before coding.

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