Datadog Coding Interview Questions
100 Datadog coding interview problems with full optimal solutions — 31 easy, 54 medium, 15 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Datadog interviewer values, and a FAQ section.
Showing 59 problems of 100
- #4easysometimes asked
4. Remove Duplicates from Sorted Array
Given a sorted array, remove duplicates in place and return the new length. Datadog uses this to test two-pointer mechanics — the same pattern they use for compacting sorted metric chunks before compression.
- #5easysometimes asked
5. Remove Element
Given an array and a value, remove all occurrences in place and return the new length. Datadog uses this as a hello-world for in-place stream filtering — same pattern as their per-tag drop-rule applied to incoming logs.
- #8easysometimes asked
8. Plus One
Given an array of digits representing an integer, increment by one and return the new digit array. Datadog uses this to gauge edge-case discipline — the 999 → 1000 case is the same shape as carry propagation in their atomic-counter rollup.
- #10easysometimes asked
10. Binary Tree Inorder Traversal
Return the inorder traversal of a binary tree's node values. Datadog asks this to test whether you can convert recursion to an explicit stack — the iterative form is required for traversing on-disk tree-indexed metric blocks where the recursion depth would blow the stack.
- #11easysometimes asked
11. Same Tree
Given two binary trees, determine if they are structurally identical with equal node values. Datadog asks this as a baseline tree-recursion question — and follows up by asking how you'd compare two ingestion-state snapshots for drift.
- #12easysometimes asked
12. Symmetric Tree
Check whether a binary tree is a mirror of itself (symmetric around its center). Datadog likes this as a paired-recursion warmup — same shape as comparing the inbound vs outbound side of a bidirectional metric pipeline.
- #13easysometimes asked
13. Maximum Depth of Binary Tree
Return the maximum depth of a binary tree. Datadog uses this as the simplest height question and then escalates to bounded-memory variants for trees stored on disk in chunked form.
- #14easysometimes asked
14. Balanced Binary Tree
Determine if a binary tree is height-balanced. Datadog asks this to test the post-order single-pass trick — return both balance status and height up the stack, avoiding O(n^2) recomputation.
- #15easysometimes asked
15. Minimum Depth of Binary Tree
Return the minimum depth — the shortest path from root to a leaf. Datadog uses this as a BFS-vs-DFS tradeoff question: DFS visits every node, BFS short-circuits at the first leaf.
- #16easyrarely asked
16. Pascal's Triangle
Generate the first N rows of Pascal's triangle. Datadog uses this as a simple DP warmup — each row depends only on the previous, the same shape as rolling-window aggregates over consecutive minutes.
- #18easysometimes asked
18. Valid Palindrome
Determine if a string is a palindrome, considering only alphanumeric characters and ignoring case. Datadog uses this as a two-pointer warmup before harder string-stream problems.
- #22easysometimes asked
22. Two Sum II - Input Array Is Sorted
Find two numbers in a sorted array that add up to a target — using O(1) memory. Datadog uses the two-pointer trick as a building block for problems on sorted metric streams where allocating a hashmap is wasteful.
- #24easysometimes asked
24. Rotate Array
Rotate an array to the right by k steps. Datadog asks this for the in-place reversal trick — same pattern as cyclically rotating a fixed-size ring buffer in their ingest pipeline.
- #25easyrarely asked
25. Reverse Bits
Reverse the bits of a 32-bit unsigned integer. Datadog uses this to test bit-twiddling and the cached-lookup-table trick — exactly the pattern they use for hash-mixing in their high-cardinality tag IDs.
- #26easysometimes asked
26. Number of 1 Bits
Count the number of 1-bits in an unsigned 32-bit integer (popcount). Datadog asks this for the Brian Kernighan trick — used in their bitmap-based tag-presence checks where popcount is in the hot path.
- #27easyrarely asked
27. Happy Number
Determine if a number is 'happy' — repeated sum of squares of digits eventually reaches 1. Datadog asks this as a cycle-detection-on-implicit-graphs question, where Floyd's algorithm applies even without a literal linked list.
- #28easyrarely asked
28. Isomorphic Strings
Determine if two strings are isomorphic — every character in s maps to one in t under a consistent 1-to-1 substitution. Datadog asks this for the two-way-mapping insight — same shape as their tag-name canonicalization on ingest.
- #33mediumsometimes asked
33. Longest Palindromic Substring
Find the longest palindromic substring. Datadog asks this for the expand-around-center pattern — O(n^2) time with O(1) space, a sweet spot for streaming validators that can't afford O(n) extra memory.
- #34mediumsometimes asked
34. Container With Most Water
Find two heights that form a container holding the most water. Datadog asks this for the two-pointer-greedy proof — the same kind of monotone-invariant reasoning needed for windowed peak detection on metric streams.
- #36mediumsometimes asked
36. Letter Combinations of a Phone Number
Generate all possible letter combinations for a phone-keypad number. Datadog uses this as a backtracking warmup — same recursive-emission structure as their downsampling rollup that enumerates resolution buckets.
- #37mediumsometimes asked
37. Remove Nth Node From End of List
Remove the Nth-from-end node in a singly linked list in one pass. Datadog tests the two-pointer-offset trick — the same pattern they use to maintain a fixed-lag window over a streaming sequence.
- #38mediumsometimes asked
38. Generate Parentheses
Generate all combinations of n well-formed parentheses. Datadog asks this for the backtracking constraint-pruning pattern — the same idea applied to enumerating valid log-parser states.
- #39mediumrarely asked
39. Swap Nodes in Pairs
Swap every two adjacent nodes in a linked list, in place. Datadog uses this to test pointer-manipulation discipline before harder variants like reverse-in-groups.
- #40mediumrarely asked
40. Next Permutation
Rearrange numbers into the lexicographically next greater permutation. Datadog uses this as a deep array-manipulation question — the algorithmic clarity is similar to the cursor-advance logic in their iterator-based query layer.
- #43mediumrarely asked
43. Valid Sudoku
Validate a 9x9 Sudoku board against the row/col/box rules in a single pass. Datadog asks this because the constraint-encoding trick (one Set per row/col/box) is the same shape as validating multi-dimensional metric tags in a single ingestion pass.
- #44mediumsometimes asked
44. Combination Sum
Find all unique combinations of candidate integers that sum to target, with reuse allowed. Datadog asks this for the backtracking-with-reuse pattern — same shape as expanding multi-resolution rollup partitions.
- #45mediumsometimes asked
45. Permutations
Generate all permutations of a distinct-integer array. Datadog uses this as a baseline backtracking question — and follows up by asking how to lazily yield permutations as an iterator over a streaming consumer.
- #46mediumsometimes asked
46. Rotate Image
Rotate an N x N matrix 90 degrees clockwise in place. Datadog tests this for the transpose-plus-reverse trick — same shape as their column-store to row-store flip during chunk compaction.
- #48mediumrarely asked
48. Spiral Matrix
Traverse an M x N matrix in spiral (clockwise) order. Datadog uses this as a 2D-traversal warmup to test boundary-management discipline — the same shape as iterating over a chunked time-series block with varying boundaries.
- #51mediumsometimes asked
51. Unique Paths
Count paths from top-left to bottom-right of an m x n grid, moving only right or down. Datadog asks this as a DP-fundamentals warmup before harder grid problems with constraints.
- #52mediumsometimes asked
52. Minimum Path Sum
Find the minimum-cost path from top-left to bottom-right of a grid, moving only right or down. Datadog uses this as a weighted-DP foundation before harder pathfinding questions.
- #53mediumsometimes asked
53. Simplify Path
Convert a Unix-style absolute path into its canonical form. Datadog uses this for the stack-based path-segment pattern — same shape as their tag-prefix normalization for hierarchical metric names.
- #54mediumsometimes asked
54. Edit Distance
Compute the minimum number of insert/delete/replace operations to convert one string into another (Levenshtein distance). Datadog asks this for the classic 2D DP — same recurrence they use when diffing two metric-name versions during schema migration.
- #55mediumrarely asked
55. Set Matrix Zeroes
Given an m x n matrix, if an element is 0, set its entire row and column to 0 — in place, O(1) extra space. Datadog asks this for the encode-state-in-matrix trick — same shape as their in-place chunk-level invalidation marks during compaction.
- #56mediumsometimes asked
56. Search a 2D Matrix
Search for a target in a row-sorted matrix where the first element of each row > the last element of the previous row. Datadog asks this to verify you spot the flatten-to-1D trick — same shape as indexing a packed sorted block as if it were a flat array.
- #58mediumsometimes asked
58. Subsets
Return all subsets of a distinct-integer array (the power set). Datadog uses this as a backtracking foundation, and probes you to discuss bitmask enumeration as an alternative.
- #59mediumsometimes asked
59. Word Search
Determine if a word exists in a 2D board where adjacent cells form a path. Datadog uses this for the DFS-with-backtracking pattern — same shape as searching for a metric-name pattern in a hierarchical store.
- #60mediumsometimes asked
60. Decode Ways
Count the number of ways to decode a digit string into letters (A=1, B=2, ..., Z=26). Datadog asks this for the 1D DP pattern — same shape as their dynamic-parser that branches on ambiguous log-line prefixes.
- #63mediumsometimes asked
63. Binary Tree Zigzag Level Order Traversal
Return a binary tree's level-order traversal but alternate left-to-right and right-to-left per level. Datadog uses this BFS variant to test that you can compose direction-flipping on top of the level-batch primitive.
- #64mediumsometimes asked
64. Construct Binary Tree from Preorder and Inorder Traversal
Reconstruct a binary tree given its preorder and inorder traversals. Datadog uses this to test the divide-and-conquer recursion pattern with O(1) lookups via a position index — same shape as deserialization in their distributed metric storage layer.
- #65easysometimes asked
65. Convert Sorted Array to Binary Search Tree
Convert a sorted array into a height-balanced BST. Datadog uses this for the recursive midpoint split — same pattern they use when bulk-loading an ordered chunk into a balanced index tree.
- #66mediumsometimes asked
66. Path Sum II
Find all root-to-leaf paths where the sum equals a target. Datadog uses this for backtracking with path tracking — same shape as enumerating valid traces through a service-dependency graph.
- #67hardsometimes asked
67. Word Ladder
Find the shortest transformation sequence from beginWord to endWord, changing one letter at a time, using only words from a dictionary. Datadog uses this as a BFS-on-implicit-graph question — same pattern they use for shortest-path queries on dynamic service topologies.
- #69mediumsometimes asked
69. Copy List with Random Pointer
Deep-copy a linked list where each node has both a next pointer and a random pointer to any node. Datadog uses this for the old-to-new mapping trick — same shape as their snapshot logic for a graph with arbitrary edges.
- #70mediumsometimes asked
70. Word Break
Given a string and a dictionary, determine if the string can be segmented into a sequence of dictionary words. Datadog uses this for the 1D DP segmentation pattern — same shape as their tokenizer for tag-pattern matching on hierarchical metric names.
- #72mediumsometimes asked
72. Sort List
Sort a linked list in O(n log n) time and O(1) extra space. Datadog asks this for the bottom-up merge-sort pattern — same shape as their external-sort routine over chunked metric files.
- #73mediumsometimes asked
73. Maximum Product Subarray
Find the contiguous subarray with the largest product. Datadog asks this for the dual-track DP trick (track both min and max) — same shape as anomaly detection that must handle bidirectional outliers.
- #75mediumsometimes asked
75. Find Peak Element
Find any peak element in O(log n). Datadog asks this for the non-monotonic binary-search pattern — peak detection in metric streams uses the same uphill-direction logic.
- #76mediumsometimes asked
76. Find the Duplicate Number
Find the single duplicate in an array of n+1 integers where each is in [1, n]. Datadog asks this for the Floyd-tortoise-on-implicit-cycle trick — finding a duplicate metric ID in O(n) time, O(1) memory.
- #82mediumsometimes asked
82. House Robber
Max-sum subsequence with no two adjacent elements. Datadog uses this as the simplest 1D DP question — same shape as their non-overlapping window aggregation over a metric stream.
- #86mediumsometimes asked
86. Design Add and Search Words Data Structure
Design a dictionary with addWord and search where '.' matches any letter. Datadog uses this for the Trie + DFS combo — same shape as their wildcard-tag-pattern lookup in the metric query layer.
- #90hardsometimes asked
90. Median of Two Sorted Arrays
Find the median of two sorted arrays in O(log(min(m,n))). Datadog asks this for the partition-based binary search — same shape as quantile estimation over two pre-aggregated metric blocks.
- #91hardsometimes asked
91. Regular Expression Matching
Implement regex matching with '.' and '*'. Datadog asks this as a hard DP — same shape as their tag-pattern engine that matches user-specified wildcards against high-cardinality metric names.
- #93hardsometimes asked
93. Reverse Nodes in k-Group
Reverse linked list nodes in groups of K, leaving the tail intact if it's shorter than K. Datadog uses this to test deep pointer manipulation — same shape as in-place chunk reversal in their reverse-time-order query optimization.
- #94hardrarely asked
94. Substring with Concatenation of All Words
Find all starting indices of substrings that are a concatenation of each word exactly once. Datadog asks this for the hash-based sliding window — same shape as multi-tag-pattern detection on a log stream.
- #95hardrarely asked
95. Longest Valid Parentheses
Find the length of the longest valid (well-formed) parentheses substring. Datadog uses this for the stack-tracking-indices trick — same shape as their balanced-segment detector on partially-corrupted log streams.
- #96hardrarely asked
96. Sudoku Solver
Solve a 9x9 Sudoku via backtracking. Datadog uses this as a deep constraint-satisfaction question — same shape as their resource-allocation solver that fills a constrained schedule.
- #97hardsometimes asked
97. First Missing Positive
Find the smallest missing positive integer in O(n) time and O(1) space. Datadog uses this for the index-as-hash trick — same shape as their dense-key occupancy check across a metric-ID space.
- #99hardrarely asked
99. N-Queens
Place N queens on an N x N board such that none attack each other. Datadog uses this as the canonical backtracking + constraint encoding question — same shape as multi-dim CSP they use for capacity placement.
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