String Questions at PhonePe: What to Expect

String manipulation is a core skill for PhonePe’s engineering roles, especially in payments, transaction processing, and data validation systems. With 20 out of 102 total coding questions focused on strings, the company clearly prioritizes this domain. Why? Because real-world financial operations—parsing UPI IDs, validating account numbers, masking sensitive data, processing transaction logs, and handling text-based protocols—are fundamentally built on efficient string operations. A candidate’s ability to clean, transform, and match string data directly impacts performance, security, and reliability at scale.

What to Expect — Types of Problems

PhonePe’s string problems typically fall into a few key categories:

• Pattern Matching & Validation: Checking formats (like UPI user@bank), verifying palindromes, or implementing wildcard matching.
• String Transformation & Encoding: Tasks like run-length encoding, URL shortening, or masking sensitive information (e.g., showing only last 4 digits).
• Substring & Window Problems: Finding longest substrings without repeating characters, minimum window containing all characters, or substring anagrams—common in log analysis.
• Parsing & Splitting: Extracting data from structured strings (CSV, logs, queries) or evaluating expressions.
• String Operations in Data Structures: Using tries for autocomplete, hash maps for frequency counts, or stacks for nested validation.

Problems often combine string manipulation with core algorithms (sliding window, two pointers, dynamic programming) and emphasize edge cases like empty strings, Unicode, and large inputs.

How to Prepare — Study Tips with One Code Example

Master the fundamentals: know how your language handles immutability (Java/Python) and character access. Practice on paper to avoid IDE reliance. Focus on time/space complexity—PhonePe expects optimal solutions for large datasets. A key pattern is the Sliding Window for substring problems. Below is an example for finding the longest substring without repeating characters:

def length_of_longest_substring(s: str) -> int:
    char_index = {}
    left = max_length = 0
    for right, ch in enumerate(s):
        if ch in char_index and char_index[ch] >= left:
            left = char_index[ch] + 1
        char_index[ch] = right
        max_length = max(max_length, right - left + 1)
    return max_length

This pattern uses a hash map to track the last seen index of each character and adjusts the window start (left) when a repeat is found within the current window. It runs in O(n) time.

Recommended Practice Order

1. Start with basic operations: reversal, anagrams, palindrome checks.
2. Move to two-pointer techniques for in-place modifications or comparisons.
3. Master sliding window for substring and window-based problems.
4. Tackle parsing problems using stacks or state machines.
5. Practice advanced problems involving dynamic programming (e.g., edit distance) or tries.
6. Always test with edge cases: empty string, single character, all same characters, large input.

Consistency is key—solve at least one string problem daily, focusing on clarity and optimization.

Practice String at PhonePe