CoinDCX Interview — DSA + System Design | External Expert Round

Company: CoinDCX Role: SDE-3 (backend) Round: Technical (DSA + System Design) Date: 11th April 2026 Type: External Expert Interview Difficulty: Medium-Hard # Context Had a tech

Company: CoinDCX Role: SDE-3 (backend) Round: Technical (DSA + System Design) Date: 11th April 2026 Type: External Expert Interview Difficulty: Medium-Hard # Context Had a technical interview with CoinDCX today. The round was conducted by an external expert (not an internal CoinDCX engineer). It covered 2 DSA questions followed by a System Design round. Sharing the questions for anyone prepping. # Question 1 — Minimum Insertions to Make a String Palindrome Problem: Given a string s, find the minimum number of steps to convert it into a palindrome. At each step, you can insert any character at any position in the string. Example: Input: "mbadm" Output: 2 Approach: This is a classic DP problem. The minimum insertions needed = len(s) - LPS(s), where LPS is the Longest Palindromic Subsequence. LPS itself can be found using LCS of s and reverse(s). # Question 2 — Minimum Window Substring Problem: Given strings S1 and S2, find the minimum substring from S1 which contains all characters from S2. Example: Input: S1 = "ADOBECODETAPK", S2 = "ATK" Output: "TAPK" Constraints: * S2 can be null/empty (edge case to handle) Approach: Classic sliding window problem. Maintain a frequency map of S2 characters, expand the window until all characters are covered, then shrink from the left to minimize. # Question 3 — Design YouTube Problem: Design YouTube at scale, serving millions of users. Key features to cover: * Upload videos * Live streaming (this was the main focus area) * Search videos * Play/stream videos The interviewer was particularly interested in how live streaming would work — ingestion pipeline, transcoding in real-time, CDN distribution, latency considerations (HLS/DASH), chat sync, etc. Hope this helps anyone prepping for CoinDCX or similar interviews. Good luck!