String Questions at Paytm: What to Expect

String manipulation is a core skill for Paytm engineers because their business—payments, e-commerce, and financial services—relies heavily on processing and validating textual data. Transactions involve parsing payment gateways, validating user inputs like account numbers or UPI IDs, sanitizing logs, and handling massive volumes of structured and unstructured text data efficiently. Out of 29 total coding questions Paytm uses in interviews, 8 are String-based, making it the single largest topic category. Mastering Strings isn't just about solving an interview problem; it directly reflects your ability to handle real-world data processing tasks critical to their platform.

What to Expect — Types of Problems

Paytm’s String questions typically focus on practical, pattern-driven problems rather than abstract puzzles. You can expect these categories:

1. Validation & Parsing: Checking if a string is a valid phone number, email, transaction ID, or follows a specific pattern (e.g., alphanumeric codes). This tests your attention to detail and knowledge of regular expressions or iterative scanning.
2. Anagram & Palindrome Variations: Common problems like checking for anagrams, finding all anagrams in a string, or determining if a string can be rearranged into a palindrome. These assess your grasp of frequency counting and efficient data structures.
3. Substring & Window Sliding: Finding the longest substring without repeating characters, substrings with exactly K distinct characters, or minimum window substring. These are high-frequency questions that test your optimization skills for Paytm's scale.
4. String Transformation: Problems involving edits (insert, delete, replace), string compression, or encoding/decoding patterns. These evaluate your ability to manipulate data with minimal memory overhead.

How to Prepare — Study Tips with One Code Example

Focus on patterns, not memorization. For each category above, learn the core algorithm and implement it in your primary language. Use a frequency map (hash map) for anagram and character count problems. Master the sliding window technique for substring problems. Always clarify edge cases (empty string, case sensitivity, special characters) before coding.

A key pattern is the Sliding Window for finding the longest substring with unique characters. Here’s how it works across languages:

def longest_unique_substring(s: str) -> int:
    char_index = {}
    left = 0
    max_len = 0

    for right, ch in enumerate(s):
        # If duplicate found, move left pointer
        if ch in char_index and char_index[ch] >= left:
            left = char_index[ch] + 1
        char_index[ch] = right
        max_len = max(max_len, right - left + 1)

    return max_len

Recommended Practice Order

1. Start with frequency counting problems (anagrams, palindrome checks).
2. Move to two-pointer techniques for in-place manipulations or palindrome expansions.
3. Practice sliding window problems for substrings, as they are highly frequent.
4. Tackle dynamic programming for edit distance and complex transformations.
5. Finally, solve parsing/validation problems using both iterative logic and regex.

Consistently time yourself and aim for a 15-20 minute solve time per problem, including edge case handling and complexity analysis.

Practice String at Paytm