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 54 problems of 100
- #31mediumfrequently asked
31. Add Two Numbers
Add two non-negative integers represented as reversed-order linked lists. Datadog uses this to test carry-propagation logic — the same logic as their atomic-counter rollup across pipeline stages.
- #32mediumfrequently asked
32. Longest Substring Without Repeating Characters
Find the length of the longest substring with no repeating characters. Datadog asks this as the sliding-window foundation — the same pattern they use for windowed deduplication of streaming events.
- #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.
- #35mediumfrequently asked
35. 3Sum
Find all unique triplets that sum to zero. Datadog uses this as the cornerstone two-pointer + dedup question — the same pattern needed for finding triple-correlations in cross-metric anomaly detection.
- #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.
- #41mediumfrequently asked
41. Search in Rotated Sorted Array
Search for a target in a rotated sorted array in O(log n). Datadog asks this because their TSDB indexes can be rotated chunks (after compaction crosses a boundary), and bisect must still work correctly.
- #42mediumfrequently asked
42. Find First and Last Position of Element in Sorted Array
Find the first and last index of a target in a sorted array using O(log n). Datadog asks this because their TSDB needs range-bisect (start and end of a value range) for every query that filters by timestamp.
- #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.
- #47mediumfrequently asked
47. Group Anagrams
Group strings that are anagrams of each other. Datadog asks this because the canonicalize-then-hash pattern is identical to how they group equivalent metric series by sorted-tag-keys for query deduplication.
- #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.
- #49mediumfrequently asked
49. Jump Game
Determine if you can reach the last index given an array of max jump lengths. Datadog asks this for the greedy max-reach pattern — same shape as a streaming reachability check over a partially-observed graph.
- #50mediumfrequently asked
50. Merge Intervals
Merge all overlapping intervals. Datadog asks this constantly because it's the foundation of compaction — merging overlapping metric chunks, log batches, or active session windows is the same problem.
- #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.
- #57mediumfrequently asked
57. Sort Colors
Sort an array of 0s, 1s, and 2s in place in one pass. Datadog asks this as the canonical Dutch National Flag warmup — same three-way-partition trick they use for tagging metric series by priority.
- #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.
- #61mediumfrequently asked
61. Validate Binary Search Tree
Determine if a binary tree satisfies BST properties. Datadog asks this for the bounds-passing recursion pattern — same shape as validating an ordered range invariant on a hierarchical metric store.
- #62mediumfrequently asked
62. Binary Tree Level Order Traversal
Return a binary tree's level-order traversal grouped by level. Datadog uses this as the BFS-foundation question — same level-batched pattern they use for paginated tree expansion in their query layer.
- #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.
- #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.
- #68mediumfrequently asked
68. Longest Consecutive Sequence
Find the longest sequence of consecutive integers in an unsorted array in O(n). Datadog asks this for the start-only expansion trick — same pattern as compacting consecutive timestamps in a sparse metric block.
- #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.
- #71mediumfrequently asked
71. LRU Cache
Design a cache with get and put operations in O(1). Datadog asks this constantly — every metric ingestion pipeline has an LRU at some boundary for resolving high-cardinality tag IDs to interned values.
- #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.
- #74mediumfrequently asked
74. Find Minimum in Rotated Sorted Array
Find the minimum value in a rotated sorted array in O(log n). Datadog asks this because their TSDB rotates compacted chunks across the time axis, and locating the pivot is the prerequisite for any range query.
- #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.
- #77mediumfrequently asked
77. Kth Largest Element in an Array
Find the kth largest element in an unsorted array. Datadog asks this for the min-heap-of-size-k trick — same streaming aggregate they use to maintain top-K leaderboards over high-cardinality metric streams.
- #78mediumfrequently asked
78. Course Schedule
Determine if you can finish all courses given prerequisite pairs — equivalent to detecting a cycle in a directed graph. Datadog asks this because their dashboard-dependency graph and metric-rollup DAG must remain acyclic.
- #79mediumfrequently asked
79. Implement Trie (Prefix Tree)
Implement a Trie with insert, search, and startsWith. Datadog uses this as the gateway to prefix-search problems — same shape as their tag-name autocomplete that serves their high-cardinality search UI.
- #80mediumfrequently asked
80. Minimum Size Subarray Sum
Find the minimum length contiguous subarray whose sum is at least target. Datadog asks this for the variable-size sliding-window pattern — same shape as their alert-window optimization over a metric stream.
- #81mediumfrequently asked
81. Number of Islands
Count connected components of '1's in a 2D grid. Datadog uses this as the canonical grid-BFS/DFS question — same shape as counting active service clusters in a service-mesh observability view.
- #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.
- #83mediumfrequently asked
83. Coin Change
Find the minimum number of coins to make a target amount. Datadog asks this for the unbounded-knapsack DP pattern — same shape as their resolution-stitching algorithm that selects the minimum number of pre-aggregated buckets to cover a query window.
- #84mediumfrequently asked
84. Longest Increasing Subsequence
Find the length of the longest strictly increasing subsequence. Datadog asks this for the patience-sorting O(n log n) trick — same shape as their monotonic-history compression for time-series anomaly trend detection.
- #85mediumfrequently asked
85. Top K Frequent Elements
Return the k most frequent elements. Datadog asks this for the heap-of-size-k + bucket-sort alternative — same shape as their top-K leaderboard for hottest metric series in a high-cardinality 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.
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